Active Record Associations

1 Why Associations?

In Rails, an association is a connection between two Active Record models. Why do we need associations between models? Because they make common operations simpler and easier in your code.

For example, consider a simple Rails application that includes a model for authors and a model for books. Each author can have many books.

Without associations, the model declarations would look like this:

class Author < ApplicationRecord
end

class Book < ApplicationRecord
end

Now, suppose we wanted to add a new book for an existing author. We'd need to do something like this:

@book = Book.create(published_at: Time.now, author_id: @author.id)

Or consider deleting an author, and ensuring that all of its books get deleted as well:

@books = Book.where(author_id: @author.id)
@books.each do |book|
  book.destroy
end
@author.destroy

With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up authors and books:

class Author < ApplicationRecord
  has_many :books, dependent: :destroy
end

class Book < ApplicationRecord
  belongs_to :author
end

With this change, creating a new book for a particular author is easier:

@book = @author.books.create(published_at: Time.now)

Deleting an author and all of its books is much easier:

@author.destroy

To learn more about the different types of associations, read the next section of this guide. That's followed by some tips and tricks for working with associations, and then by a complete reference to the methods and options for associations in Rails.

2 The Types of Associations

Rails supports six types of associations, each with a particular use-case in mind.

Here is a list of all of the supported types with a link to their API docs for more detailed information on how to use them, their method parameters, etc.

Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model belongs_to another, you instruct Rails to maintain Primary Key-Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model.

In the remainder of this guide, you'll learn how to declare and use the various forms of associations. But first, a quick introduction to the situations where each association type is appropriate.

2.1 The `belongs_to` Association

A belongs_to association sets up a connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes authors and books, and each book can be assigned to exactly one author, you'd declare the book model this way:

class Book < ApplicationRecord
  belongs_to :author
end

belongs_to Association Diagram

belongs_to associations must use the singular term. If you used the pluralized form in the above example for the author association in the Book model and tried to create the instance by Book.create(authors: @author), you would be told that there was an "uninitialized constant Book::Authors". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too.

The corresponding migration might look like this:

class CreateBooks < ActiveRecord::Migration[7.1]
  def change
    create_table :authors do |t|
      t.string :name
      t.timestamps
    end

    create_table :books do |t|
      t.belongs_to :author
      t.datetime :published_at
      t.timestamps
    end
  end
end

When used alone, belongs_to produces a one-directional one-to-one connection. Therefore each book in the above example "knows" its author, but the authors don't know about their books. To setup a bi-directional association - use belongs_to in combination with a has_one or has_many on the other model, in this case the Author model.

belongs_to does not ensure reference consistency if optional is set to true, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:

create_table :books do |t|
  t.belongs_to :author, foreign_key: true
  # ...
end

2.2 The `has_one` Association

A has_one association indicates that one other model has a reference to this model. That model can be fetched through this association.

For example, if each supplier in your application has only one account, you'd declare the supplier model like this:

class Supplier < ApplicationRecord
  has_one :account
end

The main difference from belongs_to is that the link column supplier_id is located in the other table:

has_one Association Diagram

The corresponding migration might look like this:

class CreateSuppliers < ActiveRecord::Migration[7.1]
  def change
    create_table :suppliers do |t|
      t.string :name
      t.timestamps
    end

    create_table :accounts do |t|
      t.belongs_to :supplier
      t.string :account_number
      t.timestamps
    end
  end
end

Depending on the use case, you might also need to create a unique index and/or a foreign key constraint on the supplier column for the accounts table. In this case, the column definition might look like this:

create_table :accounts do |t|
  t.belongs_to :supplier, index: { unique: true }, foreign_key: true
  # ...
end

This relation can be bi-directional when used in combination with belongs_to on the other model.

2.3 The `has_many` Association

A has_many association is similar to has_one, but indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a belongs_to association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing authors and books, the author model could be declared like this:

class Author < ApplicationRecord
  has_many :books
end

The name of the other model is pluralized when declaring a has_many association.

has_many Association Diagram

The corresponding migration might look like this:

class CreateAuthors < ActiveRecord::Migration[7.1]
  def change
    create_table :authors do |t|
      t.string :name
      t.timestamps
    end

    create_table :books do |t|
      t.belongs_to :author
      t.datetime :published_at
      t.timestamps
    end
  end
end

Depending on the use case, it's usually a good idea to create a non-unique index and optionally a foreign key constraint on the author column for the books table:

create_table :books do |t|
  t.belongs_to :author, index: true, foreign_key: true
  # ...
end

2.4 The `has_many :through` Association

A has_many :through association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:

class Physician < ApplicationRecord
  has_many :appointments
  has_many :patients, through: :appointments
end

class Appointment < ApplicationRecord
  belongs_to :physician
  belongs_to :patient
end

class Patient < ApplicationRecord
  has_many :appointments
  has_many :physicians, through: :appointments
end

has_many :through Association Diagram

The corresponding migration might look like this:

class CreateAppointments < ActiveRecord::Migration[7.1]
  def change
    create_table :physicians do |t|
      t.string :name
      t.timestamps
    end

    create_table :patients do |t|
      t.string :name
      t.timestamps
    end

    create_table :appointments do |t|
      t.belongs_to :physician
      t.belongs_to :patient
      t.datetime :appointment_date
      t.timestamps
    end
  end
end

The collection of join models can be managed via the has_many association methods. For example, if you assign:

physician.patients = patients

Then new join models are automatically created for the newly associated objects. If some that existed previously are now missing, then their join rows are automatically deleted.

Automatic deletion of join models is direct, no destroy callbacks are triggered.

The has_many :through association is also useful for setting up "shortcuts" through nested has_many associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:

class Document < ApplicationRecord
  has_many :sections
  has_many :paragraphs, through: :sections
end

class Section < ApplicationRecord
  belongs_to :document
  has_many :paragraphs
end

class Paragraph < ApplicationRecord
  belongs_to :section
end

With through: :sections specified, Rails will now understand:

@document.paragraphs

2.5 The `has_one :through` Association

A has_one :through association sets up a one-to-one connection with another model. This association indicates that the declaring model can be matched with one instance of another model by proceeding through a third model. For example, if each supplier has one account, and each account is associated with one account history, then the supplier model could look like this:

class Supplier < ApplicationRecord
  has_one :account
  has_one :account_history, through: :account
end

class Account < ApplicationRecord
  belongs_to :supplier
  has_one :account_history
end

class AccountHistory < ApplicationRecord
  belongs_to :account
end

has_one :through Association Diagram

The corresponding migration might look like this:

class CreateAccountHistories < ActiveRecord::Migration[7.1]
  def change
    create_table :suppliers do |t|
      t.string :name
      t.timestamps
    end

    create_table :accounts do |t|
      t.belongs_to :supplier
      t.string :account_number
      t.timestamps
    end

    create_table :account_histories do |t|
      t.belongs_to :account
      t.integer :credit_rating
      t.timestamps
    end
  end
end

2.6 The `has_and_belongs_to_many` Association

A has_and_belongs_to_many association creates a direct many-to-many connection with another model, with no intervening model. This association indicates that each instance of the declaring model refers to zero or more instances of another model. For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:

class Assembly < ApplicationRecord
  has_and_belongs_to_many :parts
end

class Part < ApplicationRecord
  has_and_belongs_to_many :assemblies
end

has_and_belongs_to_many Association Diagram

The corresponding migration might look like this:

class CreateAssembliesAndParts < ActiveRecord::Migration[7.1]
  def change
    create_table :assemblies do |t|
      t.string :name
      t.timestamps
    end

    create_table :parts do |t|
      t.string :part_number
      t.timestamps
    end

    create_table :assemblies_parts, id: false do |t|
      t.belongs_to :assembly
      t.belongs_to :part
    end
  end
end

2.7 Choosing Between `belongs_to` and `has_one`

If you want to set up a one-to-one relationship between two models, you'll need to add belongs_to to one, and has_one to the other. How do you know which is which?

The distinction is in where you place the foreign key (it goes on the table for the class declaring the belongs_to association), but you should give some thought to the actual meaning of the data as well. The has_one relationship says that one of something is yours - that is, that something points back to you. For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. This suggests that the correct relationships are like this:

class Supplier < ApplicationRecord
  has_one :account
end

class Account < ApplicationRecord
  belongs_to :supplier
end

The corresponding migration might look like this:

class CreateSuppliers < ActiveRecord::Migration[7.1]
  def change
    create_table :suppliers do |t|
      t.string :name
      t.timestamps
    end

    create_table :accounts do |t|
      t.bigint  :supplier_id
      t.string  :account_number
      t.timestamps
    end

    add_index :accounts, :supplier_id
  end
end

Using t.bigint :supplier_id makes the foreign key naming obvious and explicit. In current versions of Rails, you can abstract away this implementation detail by using t.references :supplier instead.

2.8 Choosing Between `has_many :through` and `has_and_belongs_to_many`

Rails offers two different ways to declare a many-to-many relationship between models. The first way is to use has_and_belongs_to_many, which allows you to make the association directly:

class Assembly < ApplicationRecord
  has_and_belongs_to_many :parts
end

class Part < ApplicationRecord
  has_and_belongs_to_many :assemblies
end

The second way to declare a many-to-many relationship is to use has_many :through. This makes the association indirectly, through a join model:

class Assembly < ApplicationRecord
  has_many :manifests
  has_many :parts, through: :manifests
end

class Manifest < ApplicationRecord
  belongs_to :assembly
  belongs_to :part
end

class Part < ApplicationRecord
  has_many :manifests
  has_many :assemblies, through: :manifests
end

The simplest rule of thumb is that you should set up a has_many :through relationship if you need to work with the relationship model as an independent entity. If you don't need to do anything with the relationship model, it may be simpler to set up a has_and_belongs_to_many relationship (though you'll need to remember to create the joining table in the database).

You should use has_many :through if you need validations, callbacks, or extra attributes on the join model.

While has_and_belongs_to_many suggests creating a join table with no primary key via id: false, consider using a composite primary key for the join table in the has_many :through relationship. For example, it's recommended to use create_table :manifests, primary_key: [:assembly_id, :part_id] in the example above.

2.9 Polymorphic Associations

A slightly more advanced twist on associations is the polymorphic association. With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model. Here's how this could be declared:

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
end

class Employee < ApplicationRecord
  has_many :pictures, as: :imageable
end

class Product < ApplicationRecord
  has_many :pictures, as: :imageable
end

You can think of a polymorphic belongs_to declaration as setting up an interface that any other model can use. From an instance of the Employee model, you can retrieve a collection of pictures: @employee.pictures.

Similarly, you can retrieve @product.pictures.

If you have an instance of the Picture model, you can get to its parent via @picture.imageable. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:

class CreatePictures < ActiveRecord::Migration[7.1]
  def change
    create_table :pictures do |t|
      t.string  :name
      t.bigint  :imageable_id
      t.string  :imageable_type
      t.timestamps
    end

    add_index :pictures, [:imageable_type, :imageable_id]
  end
end

This migration can be simplified by using the t.references form:

class CreatePictures < ActiveRecord::Migration[7.1]
  def change
    create_table :pictures do |t|
      t.string :name
      t.references :imageable, polymorphic: true
      t.timestamps
    end
  end
end

Polymorphic Association Diagram

2.10 Associations between Models with Composite Primary Keys

Rails is often able to infer the primary key - foreign key information between associated models with composite primary keys without needing extra information. Take the following example:

class Order < ApplicationRecord
  self.primary_key = [:shop_id, :id]
  has_many :books
end

class Book < ApplicationRecord
  belongs_to :order
end

Here, Rails assumes that the :id column should be used as the primary key for the association between an order and its books, just as with a regular has_many / belongs_to association. It will infer that the foreign key column on the books table is :order_id. Accessing a book's order:

order = Order.create!(id: [1, 2], status: "pending")
book = order.books.create!(title: "A Cool Book")

book.reload.order

will generate the following SQL to access the order:

SELECT * FROM orders WHERE id = 2

This only works if the model's composite primary key contains the :id column, and the column is unique for all records. In order to use the full composite primary key in associations, set the query_constraints option on the association. This option specifies a composite foreign key on the association: all columns in the foreign key will be used when querying the associated record(s). For example:

class Author < ApplicationRecord
  self.primary_key = [:first_name, :last_name]
  has_many :books, query_constraints: [:first_name, :last_name]
end

class Book < ApplicationRecord
  belongs_to :author, query_constraints: [:author_first_name, :author_last_name]
end

Accessing a book's author:

author = Author.create!(first_name: "Jane", last_name: "Doe")
book = author.books.create!(title: "A Cool Book")

book.reload.author

will use :first_name and :last_name in the SQL query:

SELECT * FROM authors WHERE first_name = 'Jane' AND last_name = 'Doe'

2.11 Self Joins

In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations:

class Employee < ApplicationRecord
  has_many :subordinates, class_name: "Employee",
                          foreign_key: "manager_id"

  belongs_to :manager, class_name: "Employee", optional: true
end

With this setup, you can retrieve @employee.subordinates and @employee.manager.

In your migrations/schema, you will add a references column to the model itself.

class CreateEmployees < ActiveRecord::Migration[7.1]
  def change
    create_table :employees do |t|
      t.references :manager, foreign_key: { to_table: :employees }
      t.timestamps
    end
  end
end

The to_table option passed to foreign_key and more are explained in SchemaStatements#add_reference.

3 Tips, Tricks, and Warnings

Here are a few things you should know to make efficient use of Active Record associations in your Rails applications:

Controlling caching
Avoiding name collisions
Updating the schema
Controlling association scope
Bi-directional associations

3.1 Controlling Caching

All of the association methods are built around caching, which keeps the result of the most recent query available for further operations. The cache is even shared across methods. For example:

# retrieves books from the database
author.books.load

# uses the cached copy of books
author.books.size

# uses the cached copy of books
author.books.empty?

But what if you want to reload the cache, because data might have been changed by some other part of the application? Just call reload on the association:

# retrieves books from the database
author.books.load

# uses the cached copy of books
author.books.size

# discards the cached copy of books and goes back to the database
author.books.reload.empty?

3.2 Avoiding Name Collisions

You are not free to use just any name for your associations. Because creating an association adds a method with that name to the model, it is a bad idea to give an association a name that is already used for an instance method of ActiveRecord::Base. The association method would override the base method and break things. For instance, attributes or connection are bad names for associations.

3.3 Updating the Schema

Associations are extremely useful, but they are not magic. You are responsible for maintaining your database schema to match your associations. In practice, this means two things, depending on what sort of associations you are creating. For belongs_to associations you need to create foreign keys, and for has_and_belongs_to_many associations you need to create the appropriate join table.

3.3.1 Creating Foreign Keys for `belongs_to` Associations

When you declare a belongs_to association, you need to create foreign keys as appropriate. For example, consider this model:

class Book < ApplicationRecord
  belongs_to :author
end

This declaration needs to be backed up by a corresponding foreign key column in the books table. For a brand new table, the migration might look something like this:

class CreateBooks < ActiveRecord::Migration[7.1]
  def change
    create_table :books do |t|
      t.datetime   :published_at
      t.string     :book_number
      t.references :author
    end
  end
end

Whereas for an existing table, it might look like this:

class AddAuthorToBooks < ActiveRecord::Migration[7.1]
  def change
    add_reference :books, :author
  end
end

If you wish to enforce referential integrity at the database level, add the foreign_key: true option to the ‘reference’ column declarations above.

3.3.2 Creating Join Tables for `has_and_belongs_to_many` Associations

If you create a has_and_belongs_to_many association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the :join_table option, Active Record creates the name by using the lexical order of the class names. So a join between author and book models will give the default join table name of "authors_books" because "a" outranks "b" in lexical ordering.

The precedence between model names is calculated using the <=> operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '_' is lexicographically less than 's' in common encodings).

Whatever the name, you must manually generate the join table with an appropriate migration. For example, consider these associations:

class Assembly < ApplicationRecord
  has_and_belongs_to_many :parts
end

class Part < ApplicationRecord
  has_and_belongs_to_many :assemblies
end

These need to be backed up by a migration to create the assemblies_parts table. This table should be created without a primary key:

class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[7.1]
  def change
    create_table :assemblies_parts, id: false do |t|
      t.bigint :assembly_id
      t.bigint :part_id
    end

    add_index :assemblies_parts, :assembly_id
    add_index :assemblies_parts, :part_id
  end
end

We pass id: false to create_table because that table does not represent a model. That's required for the association to work properly. If you observe any strange behavior in a has_and_belongs_to_many association like mangled model IDs, or exceptions about conflicting IDs, chances are you forgot that bit.

For simplicity, you can also use the method create_join_table:

class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[7.1]
  def change
    create_join_table :assemblies, :parts do |t|
      t.index :assembly_id
      t.index :part_id
    end
  end
end

3.4 Controlling Association Scope

By default, associations look for objects only within the current module's scope. This can be important when you declare Active Record models within a module. For example:

module MyApplication
  module Business
    class Supplier < ApplicationRecord
      has_one :account
    end

    class Account < ApplicationRecord
      belongs_to :supplier
    end
  end
end

This will work fine, because both the Supplier and the Account class are defined within the same scope. But the following will not work, because Supplier and Account are defined in different scopes:

module MyApplication
  module Business
    class Supplier < ApplicationRecord
      has_one :account
    end
  end

  module Billing
    class Account < ApplicationRecord
      belongs_to :supplier
    end
  end
end

To associate a model with a model in a different namespace, you must specify the complete class name in your association declaration:

module MyApplication
  module Business
    class Supplier < ApplicationRecord
      has_one :account,
        class_name: "MyApplication::Billing::Account"
    end
  end

  module Billing
    class Account < ApplicationRecord
      belongs_to :supplier,
        class_name: "MyApplication::Business::Supplier"
    end
  end
end

3.5 Bi-directional Associations

It's normal for associations to work in two directions, requiring declaration on two different models:

class Author < ApplicationRecord
  has_many :books
end

class Book < ApplicationRecord
  belongs_to :author
end

Active Record will attempt to automatically identify that these two models share a bi-directional association based on the association name. This information allows Active Record to:

Prevent needless queries for already-loaded data:

irb> author = Author.first
irb> author.books.all? do |book|
irb>   book.author.equal?(author) # No additional queries executed here
irb> end
=> true

Prevent inconsistent data (since there is only one copy of the Author object loaded):

irb> author = Author.first
irb> book = author.books.first
irb> author.name == book.author.name
=> true
irb> author.name = "Changed Name"
irb> author.name == book.author.name
=> true

Autosave associations in more cases:

irb> author = Author.new
irb> book = author.books.new
irb> book.save!
irb> book.persisted?
=> true
irb> author.persisted?
=> true

Validate the presence and absence of associations in more cases:

irb> book = Book.new
irb> book.valid?
=> false
irb> book.errors.full_messages
=> ["Author must exist"]
irb> author = Author.new
irb> book = author.books.new
irb> book.valid?
=> true

Active Record supports automatic identification for most associations with standard names. However, bi-directional associations that contain the :through or :foreign_key options will not be automatically identified.

Custom scopes on the opposite association also prevent automatic identification, as do custom scopes on the association itself unless config.active_record.automatic_scope_inversing is set to true (the default for new applications).

For example, consider the following model declarations:

class Author < ApplicationRecord
  has_many :books
end

class Book < ApplicationRecord
  belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
end

Because of the :foreign_key option, Active Record will no longer automatically recognize the bi-directional association. This can cause your application to:

Execute needless queries for the same data (in this example causing N+1 queries):

irb> author = Author.first
irb> author.books.any? do |book|
irb>   book.author.equal?(author) # This executes an author query for every book
irb> end
=> false

Reference multiple copies of a model with inconsistent data:

irb> author = Author.first
irb> book = author.books.first
irb> author.name == book.author.name
=> true
irb> author.name = "Changed Name"
irb> author.name == book.author.name
=> false

Fail to autosave associations:

irb> author = Author.new
irb> book = author.books.new
irb> book.save!
irb> book.persisted?
=> true
irb> author.persisted?
=> false

Fail to validate presence or absence:

irb> author = Author.new
irb> book = author.books.new
irb> book.valid?
=> false
irb> book.errors.full_messages
=> ["Author must exist"]

Active Record provides the :inverse_of option so you can explicitly declare bi-directional associations:

class Author < ApplicationRecord
  has_many :books, inverse_of: 'writer'
end

class Book < ApplicationRecord
  belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
end

By including the :inverse_of option in the has_many association declaration, Active Record will now recognize the bi-directional association and behave as in the initial examples above.

4 Detailed Association Reference

The following sections give the details of each type of association, including the methods that they add and the options that you can use when declaring an association.

4.1 `belongs_to` Association Reference

In database terms, the belongs_to association says that this model's table contains a column which represents a reference to another table. This can be used to set up one-to-one or one-to-many relations, depending on the setup. If the table of the other class contains the reference in a one-to-one relation, then you should use has_one instead.

4.1.1 Methods Added by `belongs_to`

When you declare a belongs_to association, the declaring class automatically gains 8 methods related to the association:

association
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
reload_association
reset_association
association_changed?
association_previously_changed?

In all of these methods, association is replaced with the symbol passed as the first argument to belongs_to. For example, given the declaration:

class Book < ApplicationRecord
  belongs_to :author
end

Each instance of the Book model will have these methods:

author
author=
build_author
create_author
create_author!
reload_author
reset_author
author_changed?
author_previously_changed?

When initializing a new has_one or belongs_to association you must use the build_ prefix to build the association, rather than the association.build method that would be used for has_many or has_and_belongs_to_many associations. To create one, use the create_ prefix.

4.1.1.1 `association`

The association method returns the associated object, if any. If no associated object is found, it returns nil.

@author = @book.author

If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), call #reload_association on the parent object.

@author = @book.reload_author

To unload the cached version of the associated object—causing the next access, if any, to query it from the database—call #reset_association on the parent object.

@book.reset_author

4.1.1.2 `association=(associate)`

The association= method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from the associated object and setting this object's foreign key to the same value.

@book.author = @author

4.1.1.3 `build_association(attributes = {})`

The build_association method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through this object's foreign key will be set, but the associated object will not yet be saved.

@author = @book.build_author(author_number: 123,
                             author_name: "John Doe")

4.1.1.4 `create_association(attributes = {})`

The create_association method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through this object's foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object will be saved.

@author = @book.create_author(author_number: 123,
                              author_name: "John Doe")

4.1.1.5 `create_association!(attributes = {})`

Does the same as create_association above, but raises ActiveRecord::RecordInvalid if the record is invalid.

4.1.1.6 `association_changed?`

The association_changed? method returns true if a new associated object has been assigned and the foreign key will be updated in the next save.

@book.author # => #<Author author_number: 123, author_name: "John Doe">
@book.author_changed? # => false

@book.author = Author.second # => #<Author author_number: 456, author_name: "Jane Smith">
@book.author_changed? # => true

@book.save!
@book.author_changed? # => false

4.1.1.7 `association_previously_changed?`

The association_previously_changed? method returns true if the previous save updated the association to reference a new associate object.

@book.author # => #<Author author_number: 123, author_name: "John Doe">
@book.author_previously_changed? # => false

@book.author = Author.second # => #<Author author_number: 456, author_name: "Jane Smith">
@book.save!
@book.author_previously_changed? # => true

4.1.2 Options for `belongs_to`

While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the belongs_to association reference. Such customizations can easily be accomplished by passing options and scope blocks when you create the association. For example, this association uses two such options:

class Book < ApplicationRecord
  belongs_to :author, touch: :books_updated_at,
    counter_cache: true
end

The belongs_to association supports these options:

:autosave
:class_name
:counter_cache
:default
:dependent
:ensuring_owner_was
:foreign_key
:foreign_type
:primary_key
:inverse_of
:optional
:polymorphic
:required
:strict_loading
:touch
:validate

4.1.2.1 `:autosave`

If you set the :autosave option to true, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting :autosave to false is not the same as not setting the :autosave option. If the :autosave option is not present, then new associated objects will be saved, but updated associated objects will not be saved.

4.1.2.2 `:class_name`

If the name of the other model cannot be derived from the association name, you can use the :class_name option to supply the model name. For example, if a book belongs to an author, but the actual name of the model containing authors is Patron, you'd set things up this way:

class Book < ApplicationRecord
  belongs_to :author, class_name: "Patron"
end

4.1.2.3 `:counter_cache`

The :counter_cache option can be used to make finding the number of belonging objects more efficient. Consider these models:

class Book < ApplicationRecord
  belongs_to :author
end

class Author < ApplicationRecord
  has_many :books
end

With these declarations, asking for the value of @author.books.size requires making a call to the database to perform a COUNT(*) query. To avoid this call, you can add a counter cache to the belonging model:

class Book < ApplicationRecord
  belongs_to :author, counter_cache: true
end

class Author < ApplicationRecord
  has_many :books
end

With this declaration, Rails will keep the cache value up to date, and then return that value in response to the size method.

Although the :counter_cache option is specified on the model that includes the belongs_to declaration, the actual column must be added to the associated (has_many) model. In the case above, you would need to add a column named books_count to the Author model.

You can override the default column name by specifying a custom column name in the counter_cache declaration instead of true. For example, to use count_of_books instead of books_count:

class Book < ApplicationRecord
  belongs_to :author, counter_cache: :count_of_books
end

class Author < ApplicationRecord
  has_many :books
end

You only need to specify the :counter_cache option on the belongs_to side of the association.

Counter cache columns are added to the owner model's list of read-only attributes through attr_readonly.

If for some reason you change the value of an owner model's primary key, and do not also update the foreign keys of the counted models, then the counter cache may have stale data. In other words, any orphaned models will still count towards the counter. To fix a stale counter cache, use reset_counters.

4.1.2.4 `:default`

When set to true, the association will not have its presence validated.

4.1.2.5 `:dependent`

If you set the :dependent option to:

:destroy, when the object is destroyed, destroy will be called on its associated objects.
:delete, when the object is destroyed, all its associated objects will be deleted directly from the database without calling their destroy method.
:destroy_async: when the object is destroyed, an ActiveRecord::DestroyAssociationAsyncJob job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work. Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner.

You should not specify this option on a belongs_to association that is connected with a has_many association on the other class. Doing so can lead to orphaned records in your database.

4.1.2.6 `:ensuring_owner_was`

Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.

4.1.2.7 `:foreign_key`

By convention, Rails assumes that the column used to hold the foreign key on this model is the name of the association with the suffix _id added. The :foreign_key option lets you set the name of the foreign key directly:

class Book < ApplicationRecord
  belongs_to :author, class_name: "Patron",
                      foreign_key: "patron_id"
end

In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.

4.1.2.8 `:foreign_type`

Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the association with a “_type” suffix. So a class that defines a belongs_to :taggable, polymorphic: true association will use “taggable_type” as the default :foreign_type.

4.1.2.9 `:primary_key`

By convention, Rails assumes that the id column is used to hold the primary key of its tables. The :primary_key option allows you to specify a different column.

For example, given we have a users table with guid as the primary key. If we want a separate todos table to hold the foreign key user_id in the guid column, then we can use primary_key to achieve this like so:

class User < ApplicationRecord
  self.primary_key = 'guid' # primary key is guid and not id
end

class Todo < ApplicationRecord
  belongs_to :user, primary_key: 'guid'
end

When we execute @user.todos.create then the @todo record will have its user_id value as the guid value of @user.

4.1.2.10 `:inverse_of`

The :inverse_of option specifies the name of the has_many or has_one association that is the inverse of this association. See the bi-directional association section for more details.

class Author < ApplicationRecord
  has_many :books, inverse_of: :author
end

class Book < ApplicationRecord
  belongs_to :author, inverse_of: :books
end

4.1.2.11 `:optional`

If you set the :optional option to true, then the presence of the associated object won't be validated. By default, this option is set to false.

4.1.2.12 `:polymorphic`

Passing true to the :polymorphic option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.

4.1.2.13 `:required`

When set to true, the association will also have its presence validated. This will validate the association itself, not the id. You can use :inverse_of to avoid an extra query during validation.

required is set to true by default and is deprecated. If you don’t want to have association presence validated, use optional: true.

4.1.2.14 `:strict_loading`

Enforces strict loading every time the associated record is loaded through this association.

4.1.2.15 `:touch`

If you set the :touch option to true, then the updated_at or updated_on timestamp on the associated object will be set to the current time whenever this object is saved or destroyed:

class Book < ApplicationRecord
  belongs_to :author, touch: true
end

class Author < ApplicationRecord
  has_many :books
end

In this case, saving or destroying a book will update the timestamp on the associated author. You can also specify a particular timestamp attribute to update:

class Book < ApplicationRecord
  belongs_to :author, touch: :books_updated_at
end

4.1.2.16 `:validate`

If you set the :validate option to true, then new associated objects will be validated whenever you save this object. By default, this is false: new associated objects will not be validated when this object is saved.

4.1.3 Scopes for `belongs_to`

There may be times when you wish to customize the query used by belongs_to. Such customizations can be achieved via a scope block. For example:

class Book < ApplicationRecord
  belongs_to :author, -> { where active: true }
end

You can use any of the standard querying methods inside the scope block. The following ones are discussed below:

where
includes
readonly
select

4.1.3.1 `where`

The where method lets you specify the conditions that the associated object must meet.

class Book < ApplicationRecord
  belongs_to :author, -> { where active: true }
end

4.1.3.2 `includes`

You can use the includes method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:

class Chapter < ApplicationRecord
  belongs_to :book
end

class Book < ApplicationRecord
  belongs_to :author
  has_many :chapters
end

class Author < ApplicationRecord
  has_many :books
end

If you frequently retrieve authors directly from chapters (@chapter.book.author), then you can make your code somewhat more efficient by including authors in the association from chapters to books:

class Chapter < ApplicationRecord
  belongs_to :book, -> { includes :author }
end

class Book < ApplicationRecord
  belongs_to :author
  has_many :chapters
end

class Author < ApplicationRecord
  has_many :books
end

There's no need to use includes for immediate associations - that is, if you have Book belongs_to :author, then the author is eager-loaded automatically when it's needed.

4.1.3.3 `readonly`

If you use readonly, then the associated object will be read-only when retrieved via the association.

4.1.3.4 `select`

The select method lets you override the SQL SELECT clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.

If you use the select method on a belongs_to association, you should also set the :foreign_key option to guarantee the correct results.

4.1.4 Do Any Associated Objects Exist?

You can see if any associated objects exist by using the association.nil? method:

if @book.author.nil?
  @msg = "No author found for this book"
end

4.1.5 When are Objects Saved?

Assigning an object to a belongs_to association does not automatically save the object. It does not save the associated object either.

4.2 `has_one` Association Reference

The has_one association creates a one-to-one match with another model. In database terms, this association says that the other class contains the foreign key. If this class contains the foreign key, then you should use belongs_to instead.

4.2.1 Methods Added by `has_one`

When you declare a has_one association, the declaring class automatically gains 6 methods related to the association:

association
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
reload_association
reset_association

In all of these methods, association is replaced with the symbol passed as the first argument to has_one. For example, given the declaration:

class Supplier < ApplicationRecord
  has_one :account
end

Each instance of the Supplier model will have these methods:

account
account=
build_account
create_account
create_account!
reload_account
reset_account

4.2.1.1 `association`

The association method returns the associated object, if any. If no associated object is found, it returns nil.

@account = @supplier.account

@account = @supplier.reload_account

To unload the cached version of the associated object—forcing the next access, if any, to query it from the database—call #reset_association on the parent object.

@supplier.reset_account

4.2.1.2 `association=(associate)`

The association= method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from this object and setting the associated object's foreign key to the same value.

@supplier.account = @account

4.2.1.3 `build_association(attributes = {})`

The build_association method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through its foreign key will be set, but the associated object will not yet be saved.

@account = @supplier.build_account(terms: "Net 30")

4.2.1.4 `create_association(attributes = {})`

The create_association method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through its foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object will be saved.

@account = @supplier.create_account(terms: "Net 30")

4.2.1.5 `create_association!(attributes = {})`

Does the same as create_association above, but raises ActiveRecord::RecordInvalid if the record is invalid.

4.2.2 Options for `has_one`

While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the has_one association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:

class Supplier < ApplicationRecord
  has_one :account, class_name: "Billing", dependent: :nullify
end

The has_one association supports these options:

:as
:autosave
:class_name
:dependent
:disable_joins
:ensuring_owner_was
:foreign_key
:inverse_of
:primary_key
:query_constraints
:required
:source
:source_type
:strict_loading
:through
:touch
:validate

4.2.2.1 `:as`

Setting the :as option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.

4.2.2.2 `:autosave`

4.2.2.3 `:class_name`

If the name of the other model cannot be derived from the association name, you can use the :class_name option to supply the model name. For example, if a supplier has an account, but the actual name of the model containing accounts is Billing, you'd set things up this way:

class Supplier < ApplicationRecord
  has_one :account, class_name: "Billing"
end

4.2.2.4 `:dependent`

Controls what happens to the associated object when its owner is destroyed:

:destroy causes the associated object to also be destroyed
:delete causes the associated object to be deleted directly from the database (so callbacks will not execute)
:destroy_async: when the object is destroyed, an ActiveRecord::DestroyAssociationAsyncJob job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work. Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner.
:nullify causes the foreign key to be set to NULL. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.
:restrict_with_exception causes an ActiveRecord::DeleteRestrictionError exception to be raised if there is an associated record
:restrict_with_error causes an error to be added to the owner if there is an associated object

It's necessary not to set or leave :nullify option for those associations that have NOT NULL database constraints. If you don't set dependent to destroy such associations you won't be able to change the associated object because the initial associated object's foreign key will be set to the unallowed NULL value.

4.2.2.5 `:disable_joins`

Specifies whether joins should be skipped for an association. If set to true, two or more queries will be generated. Note that in some cases, if order or limit is applied, it will be done in-memory due to database limitations. This option is only applicable on has_one :through associations as has_one alone does not perform a join.

4.2.2.6 `:foreign_key`

By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix _id added. The :foreign_key option lets you set the name of the foreign key directly:

class Supplier < ApplicationRecord
  has_one :account, foreign_key: "supp_id"
end

In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.

4.2.2.7 `:inverse_of`

The :inverse_of option specifies the name of the belongs_to association that is the inverse of this association. See the bi-directional association section for more details.

class Supplier < ApplicationRecord
  has_one :account, inverse_of: :supplier
end

class Account < ApplicationRecord
  belongs_to :supplier, inverse_of: :account
end

4.2.2.8 `:primary_key`

By convention, Rails assumes that the column used to hold the primary key of this model is id. You can override this and explicitly specify the primary key with the :primary_key option.

4.2.2.9 `:query_constraints`

Serves as a composite foreign key. Defines the list of columns to be used to query the associated object. This is an optional option. By default Rails will attempt to derive the value automatically. When the value is set the Array size must match associated model’s primary key or query_constraints size.

4.2.2.10 `:required`

When set to true, the association will also have its presence validated. This will validate the association itself, not the id. You can use :inverse_of to avoid an extra query during validation.

4.2.2.11 `:source`

The :source option specifies the source association name for a has_one :through association.

4.2.2.12 `:source_type`

The :source_type option specifies the source association type for a has_one :through association that proceeds through a polymorphic association.

class Author < ApplicationRecord
  has_one :book
  has_one :hardback, through: :book, source: :format, source_type: "Hardback"
  has_one :dust_jacket, through: :hardback
end

class Book < ApplicationRecord
  belongs_to :format, polymorphic: true
end

class Paperback < ApplicationRecord; end

class Hardback < ApplicationRecord
  has_one :dust_jacket
end

class DustJacket < ApplicationRecord; end

4.2.2.13 `:strict_loading`

Enforces strict loading every time the associated record is loaded through this association.

4.2.2.14 `:through`

The :through option specifies a join model through which to perform the query. has_one :through associations were discussed in detail earlier in this guide.

4.2.2.15 `:touch`

If you set the :touch option to true, then the updated_at or updated_on timestamp on the associated object will be set to the current time whenever this object is saved or destroyed:

class Supplier < ApplicationRecord
  has_one :account, touch: true
end

class Account < ApplicationRecord
  belongs_to :supplier
end

In this case, saving or destroying a supplier will update the timestamp on the associated account. You can also specify a particular timestamp attribute to update:

class Supplier < ApplicationRecord
  has_one :account, touch: :suppliers_updated_at
end

4.2.2.16 `:validate`

4.2.3 Scopes for `has_one`

There may be times when you wish to customize the query used by has_one. Such customizations can be achieved via a scope block. For example:

class Supplier < ApplicationRecord
  has_one :account, -> { where active: true }
end

You can use any of the standard querying methods inside the scope block. The following ones are discussed below:

where
includes
readonly
select

4.2.3.1 `where`

The where method lets you specify the conditions that the associated object must meet.

class Supplier < ApplicationRecord
  has_one :account, -> { where "confirmed = 1" }
end

4.2.3.2 `includes`

You can use the includes method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:

class Supplier < ApplicationRecord
  has_one :account
end

class Account < ApplicationRecord
  belongs_to :supplier
  belongs_to :representative
end

class Representative < ApplicationRecord
  has_many :accounts
end

If you frequently retrieve representatives directly from suppliers (@supplier.account.representative), then you can make your code somewhat more efficient by including representatives in the association from suppliers to accounts:

class Supplier < ApplicationRecord
  has_one :account, -> { includes :representative }
end

class Account < ApplicationRecord
  belongs_to :supplier
  belongs_to :representative
end

class Representative < ApplicationRecord
  has_many :accounts
end

4.2.3.3 `readonly`

If you use the readonly method, then the associated object will be read-only when retrieved via the association.

4.2.3.4 `select`

The select method lets you override the SQL SELECT clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.

4.2.4 Do Any Associated Objects Exist?

You can see if any associated objects exist by using the association.nil? method:

if @supplier.account.nil?
  @msg = "No account found for this supplier"
end

4.2.5 When are Objects Saved?

When you assign an object to a has_one association, that object is automatically saved (in order to update its foreign key). In addition, any object being replaced is also automatically saved, because its foreign key will change too.

If either of these saves fails due to validation errors, then the assignment statement returns false and the assignment itself is cancelled.

If the parent object (the one declaring the has_one association) is unsaved (that is, new_record? returns true) then the child objects are not saved. They will automatically when the parent object is saved.

If you want to assign an object to a has_one association without saving the object, use the build_association method.

4.3 `has_many` Association Reference

The has_many association creates a one-to-many relationship with another model. In database terms, this association says that the other class will have a foreign key that refers to instances of this class.

4.3.1 Methods Added by `has_many`

When you declare a has_many association, the declaring class automatically gains 17 methods related to the association:

In all of these methods, collection is replaced with the symbol passed as the first argument to has_many, and collection_singular is replaced with the singularized version of that symbol. For example, given the declaration:

class Author < ApplicationRecord
  has_many :books
end

Each instance of the Author model will have these methods:

books
books<<(object, ...)
books.delete(object, ...)
books.destroy(object, ...)
books=(objects)
book_ids
book_ids=(ids)
books.clear
books.empty?
books.size
books.find(...)
books.where(...)
books.exists?(...)
books.build(attributes = {}, ...)
books.create(attributes = {})
books.create!(attributes = {})
books.reload

4.3.1.1 `collection`

The collection method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation.

@books = @author.books

4.3.1.2 `collection<<(object, ...)`

The collection<< method adds one or more objects to the collection by setting their foreign keys to the primary key of the calling model.

@author.books << @book1

4.3.1.3 `collection.delete(object, ...)`

The collection.delete method removes one or more objects from the collection by setting their foreign keys to NULL.

@author.books.delete(@book1)

Additionally, objects will be destroyed if they're associated with dependent: :destroy, and deleted if they're associated with dependent: :delete_all.

4.3.1.4 `collection.destroy(object, ...)`

The collection.destroy method removes one or more objects from the collection by running destroy on each object.

@author.books.destroy(@book1)

Objects will always be removed from the database, ignoring the :dependent option.

4.3.1.5 `collection=(objects)`

The collection= method makes the collection contain only the supplied objects, by adding and deleting as appropriate. The changes are persisted to the database.

4.3.1.6 `collection_singular_ids`

The collection_singular_ids method returns an array of the ids of the objects in the collection.

@book_ids = @author.book_ids

4.3.1.7 `collection_singular_ids=(ids)`

The collection_singular_ids= method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate. The changes are persisted to the database.

4.3.1.8 `collection.clear`

The collection.clear method removes all objects from the collection according to the strategy specified by the dependent option. If no option is given, it follows the default strategy. The default strategy for has_many :through associations is delete_all, and for has_many associations is to set the foreign keys to NULL.

@author.books.clear

Objects will be deleted if they're associated with dependent: :destroy or dependent: :destroy_async, just like dependent: :delete_all.

4.3.1.9 `collection.empty?`

The collection.empty? method returns true if the collection does not contain any associated objects.

<% if @author.books.empty? %>
  No Books Found
<% end %>

4.3.1.10 `collection.size`

The collection.size method returns the number of objects in the collection.

@book_count = @author.books.size

4.3.1.11 `collection.find(...)`

The collection.find method finds objects within the collection's table.

@available_book = @author.books.find(1)

4.3.1.12 `collection.where(...)`

The collection.where method finds objects within the collection based on the conditions supplied but the objects are loaded lazily meaning that the database is queried only when the object(s) are accessed.

@available_books = @author.books.where(available: true) # No query yet
@available_book = @available_books.first # Now the database will be queried

4.3.1.13 `collection.exists?(...)`

The collection.exists? method checks whether an object meeting the supplied conditions exists in the collection's table.

4.3.1.14 `collection.build(attributes = {})`

The collection.build method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, and the link through their foreign key will be created, but the associated objects will not yet be saved.

@book = @author.books.build(published_at: Time.now,
                            book_number: "A12345")

@books = @author.books.build([
  { published_at: Time.now, book_number: "A12346" },
  { published_at: Time.now, book_number: "A12347" }
])

4.3.1.15 `collection.create(attributes = {})`

The collection.create method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, the link through its foreign key will be created, and, once it passes all of the validations specified on the associated model, the associated object will be saved.

@book = @author.books.create(published_at: Time.now,
                             book_number: "A12345")

@books = @author.books.create([
  { published_at: Time.now, book_number: "A12346" },
  { published_at: Time.now, book_number: "A12347" }
])

4.3.1.16 `collection.create!(attributes = {})`

Does the same as collection.create above, but raises ActiveRecord::RecordInvalid if the record is invalid.

4.3.1.17 `collection.reload`

The collection.reload method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation.

@books = @author.books.reload

4.3.2 Options for `has_many`

While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the has_many association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:

class Author < ApplicationRecord
  has_many :books, dependent: :delete_all, validate: false
end

The has_many association supports these options:

:as
:autosave
:class_name
:counter_cache
:dependent
:disable_joins
:ensuring_owner_was
:extend
:foreign_key
:foreign_type
:inverse_of
:primary_key
:query_constraints
:source
:source_type
:strict_loading
:through
:validate

4.3.2.1 `:as`

Setting the :as option indicates that this is a polymorphic association, as discussed earlier in this guide.

4.3.2.2 `:autosave`

4.3.2.3 `:class_name`

If the name of the other model cannot be derived from the association name, you can use the :class_name option to supply the model name. For example, if an author has many books, but the actual name of the model containing books is Transaction, you'd set things up this way:

class Author < ApplicationRecord
  has_many :books, class_name: "Transaction"
end

4.3.2.4 `:counter_cache`

This option can be used to configure a custom named :counter_cache. You only need this option when you customized the name of your :counter_cache on the belongs_to association.

4.3.2.5 `:dependent`

Controls what happens to the associated objects when their owner is destroyed:

:destroy causes all the associated objects to also be destroyed
:delete_all causes all the associated objects to be deleted directly from the database (so callbacks will not execute)
:destroy_async: when the object is destroyed, an ActiveRecord::DestroyAssociationAsyncJob job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work.
:nullify causes the foreign key to be set to NULL. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.
:restrict_with_exception causes an ActiveRecord::DeleteRestrictionError exception to be raised if there are any associated records
:restrict_with_error causes an error to be added to the owner if there are any associated objects

The :destroy and :delete_all options also affect the semantics of the collection.delete and collection= methods by causing them to destroy associated objects when they are removed from the collection.

4.3.2.6 `:disable_joins`

Specifies whether joins should be skipped for an association. If set to true, two or more queries will be generated. Note that in some cases, if order or limit is applied, it will be done in-memory due to database limitations. This option is only applicable on has_many :through associations as has_many alone do not perform a join.

4.3.2.7 `:ensuring_owner_was`

Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.

4.3.2.8 `:extend`

Specifies a module or array of modules that will be extended into the association object returned. Useful for defining methods on associations, especially when they should be shared between multiple association objects.

4.3.2.9 `:foreign_key`

class Author < ApplicationRecord
  has_many :books, foreign_key: "cust_id"
end

In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.

4.3.2.10 `:foreign_type`

Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a has_many :tags, as: :taggable association will use “taggable_type” as the default :foreign_type.

4.3.2.11 `:inverse_of`

The :inverse_of option specifies the name of the belongs_to association that is the inverse of this association. See the bi-directional association section for more details.

class Author < ApplicationRecord
  has_many :books, inverse_of: :author
end

class Book < ApplicationRecord
  belongs_to :author, inverse_of: :books
end

4.3.2.12 `:primary_key`

By convention, Rails assumes that the column used to hold the primary key of the association is id. You can override this and explicitly specify the primary key with the :primary_key option.

Let's say the users table has id as the primary_key but it also has a guid column. The requirement is that the todos table should hold the guid column value as the foreign key and not id value. This can be achieved like this:

class User < ApplicationRecord
  has_many :todos, primary_key: :guid
end

Now if we execute @todo = @user.todos.create then the @todo record's user_id value will be the guid value of @user.

4.3.2.13 `:query_constraints`

4.3.2.14 `:source`

The :source option specifies the source association name for a has_many :through association. You only need to use this option if the name of the source association cannot be automatically inferred from the association name.

4.3.2.15 `:source_type`

The :source_type option specifies the source association type for a has_many :through association that proceeds through a polymorphic association.

class Author < ApplicationRecord
  has_many :books
  has_many :paperbacks, through: :books, source: :format, source_type: "Paperback"
end

class Book < ApplicationRecord
  belongs_to :format, polymorphic: true
end

class Hardback < ApplicationRecord; end
class Paperback < ApplicationRecord; end

4.3.2.16 `:strict_loading`

When set to true, enforces strict loading every time the associated record is loaded through this association.

4.3.2.17 `:through`

The :through option specifies a join model through which to perform the query. has_many :through associations provide a way to implement many-to-many relationships, as discussed earlier in this guide.

4.3.2.18 `:validate`

If you set the :validate option to false, then new associated objects will not be validated whenever you save this object. By default, this is true: new associated objects will be validated when this object is saved.

4.3.3 Scopes for `has_many`

There may be times when you wish to customize the query used by has_many. Such customizations can be achieved via a scope block. For example:

class Author < ApplicationRecord
  has_many :books, -> { where processed: true }
end

You can use any of the standard querying methods inside the scope block. The following ones are discussed below:

where
extending
group
includes
limit
offset
order
readonly
select
distinct

4.3.3.1 `where`

The where method lets you specify the conditions that the associated object must meet.

class Author < ApplicationRecord
  has_many :confirmed_books, -> { where "confirmed = 1" },
    class_name: "Book"
end

You can also set conditions via a hash:

class Author < ApplicationRecord
  has_many :confirmed_books, -> { where confirmed: true },
    class_name: "Book"
end

If you use a hash-style where option, then record creation via this association will be automatically scoped using the hash. In this case, using @author.confirmed_books.create or @author.confirmed_books.build will create books where the confirmed column has the value true.

4.3.3.2 `extending`

The extending method specifies a named module to extend the association proxy. Association extensions are discussed in detail later in this guide.

4.3.3.3 `group`

The group method supplies an attribute name to group the result set by, using a GROUP BY clause in the finder SQL.

class Author < ApplicationRecord
  has_many :chapters, -> { group 'books.id' },
                      through: :books
end

4.3.3.4 `includes`

You can use the includes method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:

class Author < ApplicationRecord
  has_many :books
end

class Book < ApplicationRecord
  belongs_to :author
  has_many :chapters
end

class Chapter < ApplicationRecord
  belongs_to :book
end

If you frequently retrieve chapters directly from authors (@author.books.chapters), then you can make your code somewhat more efficient by including chapters in the association from authors to books:

class Author < ApplicationRecord
  has_many :books, -> { includes :chapters }
end

class Book < ApplicationRecord
  belongs_to :author
  has_many :chapters
end

class Chapter < ApplicationRecord
  belongs_to :book
end

4.3.3.5 `limit`

The limit method lets you restrict the total number of objects that will be fetched through an association.

class Author < ApplicationRecord
  has_many :recent_books,
    -> { order('published_at desc').limit(100) },
    class_name: "Book"
end

4.3.3.6 `offset`

The offset method lets you specify the starting offset for fetching objects via an association. For example, -> { offset(11) } will skip the first 11 records.

4.3.3.7 `order`

The order method dictates the order in which associated objects will be received (in the syntax used by an SQL ORDER BY clause).

class Author < ApplicationRecord
  has_many :books, -> { order "date_confirmed DESC" }
end

4.3.3.8 `readonly`

If you use the readonly method, then the associated objects will be read-only when retrieved via the association.

4.3.3.9 `select`

The select method lets you override the SQL SELECT clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.

If you specify your own select, be sure to include the primary key and foreign key columns of the associated model. If you do not, Rails will throw an error.

4.3.3.10 `distinct`

Use the distinct method to keep the collection free of duplicates. This is mostly useful together with the :through option.

class Person < ApplicationRecord
  has_many :readings
  has_many :articles, through: :readings
end

irb> person = Person.create(name: 'John')
irb> article = Article.create(name: 'a1')
irb> person.articles << article
irb> person.articles << article
irb> person.articles.to_a
=> [#<Article id: 5, name: "a1">, #<Article id: 5, name: "a1">]
irb> Reading.all.to_a
=> [#<Reading id: 12, person_id: 5, article_id: 5>, #<Reading id: 13, person_id: 5, article_id: 5>]

In the above case there are two readings and person.articles brings out both of them even though these records are pointing to the same article.

Now let's set distinct:

class Person
  has_many :readings
  has_many :articles, -> { distinct }, through: :readings
end

irb> person = Person.create(name: 'Honda')
irb> article = Article.create(name: 'a1')
irb> person.articles << article
irb> person.articles << article
irb> person.articles.to_a
=> [#<Article id: 7, name: "a1">]
irb> Reading.all.to_a
=> [#<Reading id: 16, person_id: 7, article_id: 7>, #<Reading id: 17, person_id: 7, article_id: 7>]

In the above case there are still two readings. However person.articles shows only one article because the collection loads only unique records.

If you want to make sure that, upon insertion, all of the records in the persisted association are distinct (so that you can be sure that when you inspect the association that you will never find duplicate records), you should add a unique index on the table itself. For example, if you have a table named readings and you want to make sure the articles can only be added to a person once, you could add the following in a migration:

add_index :readings, [:person_id, :article_id], unique: true

Once you have this unique index, attempting to add the article to a person twice will raise an ActiveRecord::RecordNotUnique error:

irb> person = Person.create(name: 'Honda')
irb> article = Article.create(name: 'a1')
irb> person.articles << article
irb> person.articles << article
ActiveRecord::RecordNotUnique

Note that checking for uniqueness using something like include? is subject to race conditions. Do not attempt to use include? to enforce distinctness in an association. For instance, using the article example from above, the following code would be racy because multiple users could be attempting this at the same time:

person.articles << article unless person.articles.include?(article)

4.3.4 When are Objects Saved?

When you assign an object to a has_many association, that object is automatically saved (in order to update its foreign key). If you assign multiple objects in one statement, then they are all saved.

If any of these saves fails due to validation errors, then the assignment statement returns false and the assignment itself is cancelled.

If the parent object (the one declaring the has_many association) is unsaved (that is, new_record? returns true) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.

If you want to assign an object to a has_many association without saving the object, use the collection.build method.

4.4 `has_and_belongs_to_many` Association Reference

The has_and_belongs_to_many association creates a many-to-many relationship with another model. In database terms, this associates two classes via an intermediate join table that includes foreign keys referring to each of the classes.

4.4.1 Methods Added by `has_and_belongs_to_many`

When you declare a has_and_belongs_to_many association, the declaring class automatically gains several methods related to the association:

In all of these methods, collection is replaced with the symbol passed as the first argument to has_and_belongs_to_many, and collection_singular is replaced with the singularized version of that symbol. For example, given the declaration:

class Part < ApplicationRecord
  has_and_belongs_to_many :assemblies
end

Each instance of the Part model will have these methods:

assemblies
assemblies<<(object, ...)
assemblies.delete(object, ...)
assemblies.destroy(object, ...)
assemblies=(objects)
assembly_ids
assembly_ids=(ids)
assemblies.clear
assemblies.empty?
assemblies.size
assemblies.find(...)
assemblies.where(...)
assemblies.exists?(...)
assemblies.build(attributes = {}, ...)
assemblies.create(attributes = {})
assemblies.create!(attributes = {})
assemblies.reload

4.4.1.1 Additional Column Methods

If the join table for a has_and_belongs_to_many association has additional columns beyond the two foreign keys, these columns will be added as attributes to records retrieved via that association. Records returned with additional attributes will always be read-only, because Rails cannot save changes to those attributes.

The use of extra attributes on the join table in a has_and_belongs_to_many association is deprecated. If you require this sort of complex behavior on the table that joins two models in a many-to-many relationship, you should use a has_many :through association instead of has_and_belongs_to_many.

4.4.1.2 `collection`

The collection method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation.

@assemblies = @part.assemblies

4.4.1.3 `collection<<(object, ...)`

The collection<< method adds one or more objects to the collection by creating records in the join table.

@part.assemblies << @assembly1

This method is aliased as collection.concat and collection.push.

4.4.1.4 `collection.delete(object, ...)`

The collection.delete method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.

@part.assemblies.delete(@assembly1)

4.4.1.5 `collection.destroy(object, ...)`

The collection.destroy method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.

@part.assemblies.destroy(@assembly1)

4.4.1.6 `collection=(objects)`

The collection= method makes the collection contain only the supplied objects, by adding and deleting as appropriate. The changes are persisted to the database.

4.4.1.7 `collection_singular_ids`

The collection_singular_ids method returns an array of the ids of the objects in the collection.

@assembly_ids = @part.assembly_ids

4.4.1.8 `collection_singular_ids=(ids)`

4.4.1.9 `collection.clear`

The collection.clear method removes every object from the collection by deleting the rows from the joining table. This does not destroy the associated objects.

4.4.1.10 `collection.empty?`

The collection.empty? method returns true if the collection does not contain any associated objects.

<% if @part.assemblies.empty? %>
  This part is not used in any assemblies
<% end %>

4.4.1.11 `collection.size`

The collection.size method returns the number of objects in the collection.

@assembly_count = @part.assemblies.size

4.4.1.12 `collection.find(...)`

The collection.find method finds objects within the collection's table.

@assembly = @part.assemblies.find(1)

4.4.1.13 `collection.where(...)`

@new_assemblies = @part.assemblies.where("created_at > ?", 2.days.ago)

4.4.1.14 `collection.exists?(...)`

The collection.exists? method checks whether an object meeting the supplied conditions exists in the collection's table.

4.4.1.15 `collection.build(attributes = {})`

The collection.build method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through the join table will be created, but the associated object will not yet be saved.

@assembly = @part.assemblies.build({ assembly_name: "Transmission housing" })

4.4.1.16 `collection.create(attributes = {})`

The collection.create method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through the join table will be created, and, once it passes all of the validations specified on the associated model, the associated object will be saved.

@assembly = @part.assemblies.create({ assembly_name: "Transmission housing" })

4.4.1.17 `collection.create!(attributes = {})`

Does the same as collection.create, but raises ActiveRecord::RecordInvalid if the record is invalid.

4.4.1.18 `collection.reload`

The collection.reload method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation.

@assemblies = @part.assemblies.reload

4.4.2 Options for `has_and_belongs_to_many`

While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the has_and_belongs_to_many association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies, -> { readonly },
                                       autosave: true
end

The has_and_belongs_to_many association supports these options:

:association_foreign_key
:autosave
:class_name
:foreign_key
:join_table
:strict_loading
:validate

4.4.2.1 `:association_foreign_key`

By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to the other model is the name of that model with the suffix _id added. The :association_foreign_key option lets you set the name of the foreign key directly:

The :foreign_key and :association_foreign_key options are useful when setting up a many-to-many self-join. For example:

class User < ApplicationRecord
  has_and_belongs_to_many :friends,
      class_name: "User",
      foreign_key: "this_user_id",
      association_foreign_key: "other_user_id"
end

4.4.2.2 `:autosave`

4.4.2.3 `:class_name`

If the name of the other model cannot be derived from the association name, you can use the :class_name option to supply the model name. For example, if a part has many assemblies, but the actual name of the model containing assemblies is Gadget, you'd set things up this way:

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies, class_name: "Gadget"
end

4.4.2.4 `:foreign_key`

By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to this model is the name of this model with the suffix _id added. The :foreign_key option lets you set the name of the foreign key directly:

class User < ApplicationRecord
  has_and_belongs_to_many :friends,
      class_name: "User",
      foreign_key: "this_user_id",
      association_foreign_key: "other_user_id"
end

4.4.2.5 `:join_table`

If the default name of the join table, based on lexical ordering, is not what you want, you can use the :join_table option to override the default.

4.4.2.6 `:strict_loading`

Enforces strict loading every time an associated record is loaded through this association.

4.4.2.7 `:validate`

4.4.3 Scopes for `has_and_belongs_to_many`

There may be times when you wish to customize the query used by has_and_belongs_to_many. Such customizations can be achieved via a scope block. For example:

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies, -> { where active: true }
end

You can use any of the standard querying methods inside the scope block. The following ones are discussed below:

where
extending
group
includes
limit
offset
order
readonly
select
distinct

4.4.3.1 `where`

The where method lets you specify the conditions that the associated object must meet.

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies,
    -> { where "factory = 'Seattle'" }
end

You can also set conditions via a hash:

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies,
    -> { where factory: 'Seattle' }
end

If you use a hash-style where, then record creation via this association will be automatically scoped using the hash. In this case, using @parts.assemblies.create or @parts.assemblies.build will create assemblies where the factory column has the value "Seattle".

4.4.3.2 `extending`

The extending method specifies a named module to extend the association proxy. Association extensions are discussed in detail later in this guide.

4.4.3.3 `group`

The group method supplies an attribute name to group the result set by, using a GROUP BY clause in the finder SQL.

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies, -> { group "factory" }
end

4.4.3.4 `includes`

You can use the includes method to specify second-order associations that should be eager-loaded when this association is used.

4.4.3.5 `limit`

The limit method lets you restrict the total number of objects that will be fetched through an association.

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies,
    -> { order("created_at DESC").limit(50) }
end

4.4.3.6 `offset`

The offset method lets you specify the starting offset for fetching objects via an association. For example, if you set offset(11), it will skip the first 11 records.

4.4.3.7 `order`

The order method dictates the order in which associated objects will be received (in the syntax used by an SQL ORDER BY clause).

class Parts < ApplicationRecord
  has_and_belongs_to_many :assemblies,
    -> { order "assembly_name ASC" }
end

4.4.3.8 `readonly`

If you use the readonly method, then the associated objects will be read-only when retrieved via the association.

4.4.3.9 `select`

The select method lets you override the SQL SELECT clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.

4.4.3.10 `distinct`

Use the distinct method to remove duplicates from the collection.

4.4.4 When are Objects Saved?

When you assign an object to a has_and_belongs_to_many association, that object is automatically saved (in order to update the join table). If you assign multiple objects in one statement, then they are all saved.

If any of these saves fails due to validation errors, then the assignment statement returns false and the assignment itself is cancelled.

If the parent object (the one declaring the has_and_belongs_to_many association) is unsaved (that is, new_record? returns true) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.

If you want to assign an object to a has_and_belongs_to_many association without saving the object, use the collection.build method.

4.5 Association Callbacks

Normal callbacks hook into the life cycle of Active Record objects, allowing you to work with those objects at various points. For example, you can use a :before_save callback to cause something to happen just before an object is saved.

Association callbacks are similar to normal callbacks, but they are triggered by events in the life cycle of a collection. There are four available association callbacks:

before_add
after_add
before_remove
after_remove

You define association callbacks by adding options to the association declaration. For example:

class Author < ApplicationRecord
  has_many :books, before_add: :check_credit_limit

  def check_credit_limit(book)
    # ...
  end
end

Rails passes the object being added or removed to the callback.

You can stack callbacks on a single event by passing them as an array:

class Author < ApplicationRecord
  has_many :books,
    before_add: [:check_credit_limit, :calculate_shipping_charges]

  def check_credit_limit(book)
    # ...
  end

  def calculate_shipping_charges(book)
    # ...
  end
end

If a before_add callback throws :abort, the object does not get added to the collection. Similarly, if a before_remove callback throws :abort, the object does not get removed from the collection:

# book won't be added if the limit has been reached
def check_credit_limit(book)
  throw(:abort) if limit_reached?
end

These callbacks are called only when the associated objects are added or removed through the association collection:

# Triggers `before_add` callback
author.books << book
author.books = [book, book2]

# Does not trigger the `before_add` callback
book.update(author_id: 1)

4.6 Association Extensions

You're not limited to the functionality that Rails automatically builds into association proxy objects. You can also extend these objects through anonymous modules, adding new finders, creators, or other methods. For example:

class Author < ApplicationRecord
  has_many :books do
    def find_by_book_prefix(book_number)
      find_by(category_id: book_number[0..2])
    end
  end
end

If you have an extension that should be shared by many associations, you can use a named extension module. For example:

module FindRecentExtension
  def find_recent
    where("created_at > ?", 5.days.ago)
  end
end

class Author < ApplicationRecord
  has_many :books, -> { extending FindRecentExtension }
end

class Supplier < ApplicationRecord
  has_many :deliveries, -> { extending FindRecentExtension }
end

Extensions can refer to the internals of the association proxy using these three attributes of the proxy_association accessor:

proxy_association.owner returns the object that the association is a part of.
proxy_association.reflection returns the reflection object that describes the association.
proxy_association.target returns the associated object for belongs_to or has_one, or the collection of associated objects for has_many or has_and_belongs_to_many.

4.7 Association Scoping using the Association Owner

The owner of the association can be passed as a single argument to the scope block in situations where you need even more control over the association scope. However, as a caveat, preloading the association will no longer be possible.

class Supplier < ApplicationRecord
  has_one :account, ->(supplier) { where active: supplier.active? }
end

5 Single Table Inheritance (STI)

Sometimes, you may want to share fields and behavior between different models. Let's say we have Car, Motorcycle, and Bicycle models. We will want to share the color and price fields and some methods for all of them, but having some specific behavior for each, and separated controllers too.

First, let's generate the base Vehicle model:

$ bin/rails generate model vehicle type:string color:string price:decimal{10.2}

Did you note we are adding a "type" field? Since all models will be saved in a single database table, Rails will save in this column the name of the model that is being saved. In our example, this can be "Car", "Motorcycle" or "Bicycle." STI won't work without a "type" field in the table.

Next, we will generate the Car model that inherits from Vehicle. For this, we can use the --parent=PARENT option, which will generate a model that inherits from the specified parent and without equivalent migration (since the table already exists).

For example, to generate the Car model:

$ bin/rails generate model car --parent=Vehicle

The generated model will look like this:

class Car < Vehicle
end

This means that all behavior added to Vehicle is available for Car too, as associations, public methods, etc.

Creating a car will save it in the vehicles table with "Car" as the type field:

Car.create(color: 'Red', price: 10000)

will generate the following SQL:

INSERT INTO "vehicles" ("type", "color", "price") VALUES ('Car', 'Red', 10000)

Querying car records will search only for vehicles that are cars:

Car.all

will run a query like:

SELECT "vehicles".* FROM "vehicles" WHERE "vehicles"."type" IN ('Car')

6 Delegated Types

Single Table Inheritance (STI) works best when there is little difference between subclasses and their attributes, but includes all attributes of all subclasses you need to create a single table.

The disadvantage of this approach is that it results in bloat to that table. Since it will even include attributes specific to a subclass that aren't used by anything else.

In the following example, there are two Active Record models that inherit from the same "Entry" class which includes the subject attribute.

# Schema: entries[ id, type, subject, created_at, updated_at]
class Entry < ApplicationRecord
end

class Comment < Entry
end

class Message < Entry
end

Delegated types solves this problem, via delegated_type.

In order to use delegated types, we have to model our data in a particular way. The requirements are as follows:

There is a superclass that stores shared attributes among all subclasses in it's table.
Each subclass must inherit from the super class, and will have a separate table for any additional attributes specific to it.

This eliminates the need to define attributes in a single table that are unintentionally shared among all subclasses.

In order to apply this to our example above, we need to regenerate our models. First, let's generate the base Entry model which will act as our superclass:

$ bin/rails generate model entry entryable_type:string entryable_id:integer

Then, we will generate new Message and Comment models for delegation:

$ bin/rails generate model message subject:string body:string
$ bin/rails generate model comment content:string

After running the generators, we should end up with models that look like this:

# Schema: entries[ id, entryable_type, entryable_id, created_at, updated_at ]
class Entry < ApplicationRecord
end

# Schema: messages[ id, subject, body, created_at, updated_at ]
class Message < ApplicationRecord
end

# Schema: comments[ id, content, created_at, updated_at ]
class Comment < ApplicationRecord
end

6.1 Declare `delegated_type`

First, declare a delegated_type in the superclass Entry.

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ], dependent: :destroy
end

The entryable parameter specifies the field to use for delegation, and include the types Message and Comment as the delegate classes.

The Entry class has entryable_type and entryable_id fields. This is the field with the _type, _id suffixes added to the name entryable in the delegated_type definition. entryable_type stores the subclass name of the delegatee, and entryable_id stores the record id of the delegatee subclass.

Next, we must define a module to implement those delegated types, by declaring the as: :entryable parameter to the has_one association.

module Entryable
  extend ActiveSupport::Concern

  included do
    has_one :entry, as: :entryable, touch: true
  end
end

And then include the created module in your subclass.

class Message < ApplicationRecord
  include Entryable
end

class Comment < ApplicationRecord
  include Entryable
end

With this definition complete, our Entry delegator now provides the following methods:

Method	Return
`Entry#entryable_class`	Message or Comment
`Entry#entryable_name`	"message" or "comment"
`Entry.messages`	`Entry.where(entryable_type: "Message")`
`Entry#message?`	Returns true when `entryable_type == "Message"`
`Entry#message`	Returns the message record, when `entryable_type == "Message"`, otherwise `nil`
`Entry#message_id`	Returns `entryable_id`, when `entryable_type == "Message"`, otherwise `nil`
`Entry.comments`	`Entry.where(entryable_type: "Comment")`
`Entry#comment?`	Returns true when `entryable_type == "Comment"`
`Entry#comment`	Returns the comment record, when `entryable_type == "Comment"`, otherwise `nil`
`Entry#comment_id`	Returns entryable_id, when `entryable_type == "Comment"`, otherwise `nil`

6.2 Object creation

When creating a new Entry object, we can specify the entryable subclass at the same time.

Entry.create! entryable: Message.new(subject: "hello!")

6.3 Adding further delegation

We can expand our Entry delegator and enhance it further by defining delegate and using polymorphism on the subclasses. For example, to delegate the title method from Entry to it's subclasses:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ]
  delegate :title, to: :entryable
end

class Message < ApplicationRecord
  include Entryable

  def title
    subject
  end
end

class Comment < ApplicationRecord
  include Entryable

  def title
    content.truncate(20)
  end
end

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