Active Model Basics

1 What is Active Model?

Active Model is a library containing various modules used in developing classes that need some features present on Active Record. Some of these modules are explained below.

1.1 API

ActiveModel::API adds the ability for a class to work with Action Pack and Action View right out of the box.

class EmailContact
  include ActiveModel::API

  attr_accessor :name, :email, :message
  validates :name, :email, :message, presence: true

  def deliver
    if valid?
      # deliver email
    end
  end
end

When including ActiveModel::API you get some features like:

model name introspection
conversions
translations
validations

It also gives you the ability to initialize an object with a hash of attributes, much like any Active Record object.

irb> email_contact = EmailContact.new(name: 'David', email: 'david@example.com', message: 'Hello World')
irb> email_contact.name
=> "David"
irb> email_contact.email
=> "david@example.com"
irb> email_contact.valid?
=> true
irb> email_contact.persisted?
=> false

Any class that includes ActiveModel::API can be used with form_with, render and any other Action View helper methods, just like Active Record objects.

1.2 Attribute Methods

The ActiveModel::AttributeMethods module can add custom prefixes and suffixes on methods of a class. It is used by defining the prefixes and suffixes and which methods on the object will use them.

class Person
  include ActiveModel::AttributeMethods

  attribute_method_prefix 'reset_'
  attribute_method_suffix '_highest?'
  define_attribute_methods 'age'

  attr_accessor :age

  private
    def reset_attribute(attribute)
      send("#{attribute}=", 0)
    end

    def attribute_highest?(attribute)
      send(attribute) > 100
    end
end

irb> person = Person.new
irb> person.age = 110
irb> person.age_highest?
=> true
irb> person.reset_age
=> 0
irb> person.age_highest?
=> false

ActiveModel::Callbacks gives Active Record style callbacks. This provides an ability to define callbacks which run at appropriate times. After defining callbacks, you can wrap them with before, after, and around custom methods.

class Person
  extend ActiveModel::Callbacks

  define_model_callbacks :update

  before_update :reset_me

  def update
    run_callbacks(:update) do
      # This method is called when update is called on an object.
    end
  end

  def reset_me
    # This method is called when update is called on an object as a before_update callback is defined.
  end
end

1.4 Conversion

If a class defines persisted? and id methods, then you can include the ActiveModel::Conversion module in that class, and call the Rails conversion methods on objects of that class.

class Person
  include ActiveModel::Conversion

  def persisted?
    false
  end

  def id
    nil
  end
end

irb> person = Person.new
irb> person.to_model == person
=> true
irb> person.to_key
=> nil
irb> person.to_param
=> nil

1.5 Dirty

An object becomes dirty when it has gone through one or more changes to its attributes and has not been saved. ActiveModel::Dirty gives the ability to check whether an object has been changed or not. It also has attribute-based accessor methods. Let's consider a Person class with attributes first_name and last_name:

class Person
  include ActiveModel::Dirty
  define_attribute_methods :first_name, :last_name

  def first_name
    @first_name
  end

  def first_name=(value)
    first_name_will_change!
    @first_name = value
  end

  def last_name
    @last_name
  end

  def last_name=(value)
    last_name_will_change!
    @last_name = value
  end

  def save
    # do save work...
    changes_applied
  end
end

1.5.1 Querying an Object Directly for its List of All Changed Attributes

irb> person = Person.new
irb> person.changed?
=> false

irb> person.first_name = "First Name"
irb> person.first_name
=> "First Name"

# Returns true if any of the attributes have unsaved changes.
irb> person.changed?
=> true

# Returns a list of attributes that have changed before saving.
irb> person.changed
=> ["first_name"]

# Returns a Hash of the attributes that have changed with their original values.
irb> person.changed_attributes
=> {"first_name"=>nil}

# Returns a Hash of changes, with the attribute names as the keys, and the values as an array of the old and new values for that field.
irb> person.changes
=> {"first_name"=>[nil, "First Name"]}

1.5.2 Attribute-based Accessor Methods

Track whether the particular attribute has been changed or not.

irb> person.first_name
=> "First Name"

# attr_name_changed?
irb> person.first_name_changed?
=> true

Track the previous value of the attribute.

# attr_name_was accessor
irb> person.first_name_was
=> nil

Track both previous and current values of the changed attribute. Returns an array if changed, otherwise returns nil.

# attr_name_change
irb> person.first_name_change
=> [nil, "First Name"]
irb> person.last_name_change
=> nil

1.6 Validations

The ActiveModel::Validations module adds the ability to validate objects like in Active Record.

class Person
  include ActiveModel::Validations

  attr_accessor :name, :email, :token

  validates :name, presence: true
  validates_format_of :email, with: /\A([^\s]+)((?:[-a-z0-9]\.)[a-z]{2,})\z/i
  validates! :token, presence: true
end

irb> person = Person.new
irb> person.token = "2b1f325"
irb> person.valid?
=> false
irb> person.name = 'vishnu'
irb> person.email = 'me'
irb> person.valid?
=> false
irb> person.email = 'me@vishnuatrai.com'
irb> person.valid?
=> true
irb> person.token = nil
irb> person.valid?
ActiveModel::StrictValidationFailed

1.7 Naming

ActiveModel::Naming adds several class methods which make naming and routing easier to manage. The module defines the model_name class method which will define several accessors using some ActiveSupport::Inflector methods.

class Person
  extend ActiveModel::Naming
end

Person.model_name.name                # => "Person"
Person.model_name.singular            # => "person"
Person.model_name.plural              # => "people"
Person.model_name.element             # => "person"
Person.model_name.human               # => "Person"
Person.model_name.collection          # => "people"
Person.model_name.param_key           # => "person"
Person.model_name.i18n_key            # => :person
Person.model_name.route_key           # => "people"
Person.model_name.singular_route_key  # => "person"

1.8 Model

ActiveModel::Model allows implementing models similar to ActiveRecord::Base.

class EmailContact
  include ActiveModel::Model

  attr_accessor :name, :email, :message
  validates :name, :email, :message, presence: true

  def deliver
    if valid?
      # deliver email
    end
  end
end

When including ActiveModel::Model you get all the features from ActiveModel::API.

1.9 Serialization

ActiveModel::Serialization provides basic serialization for your object. You need to declare an attributes Hash which contains the attributes you want to serialize. Attributes must be strings, not symbols.

class Person
  include ActiveModel::Serialization

  attr_accessor :name

  def attributes
    { 'name' => nil }
  end
end

Now you can access a serialized Hash of your object using the serializable_hash method.

irb> person = Person.new
irb> person.serializable_hash
=> {"name"=>nil}
irb> person.name = "Bob"
irb> person.serializable_hash
=> {"name"=>"Bob"}

1.9.1 ActiveModel::Serializers

Active Model also provides the ActiveModel::Serializers::JSON module for JSON serializing / deserializing. This module automatically includes the previously discussed ActiveModel::Serialization module.

1.9.1.1 ActiveModel::Serializers::JSON

To use ActiveModel::Serializers::JSON you only need to change the module you are including from ActiveModel::Serialization to ActiveModel::Serializers::JSON.

class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes
    { 'name' => nil }
  end
end

The as_json method, similar to serializable_hash, provides a Hash representing the model.

irb> person = Person.new
irb> person.as_json
=> {"name"=>nil}
irb> person.name = "Bob"
irb> person.as_json
=> {"name"=>"Bob"}

You can also define the attributes for a model from a JSON string. However, you need to define the attributes= method on your class:

class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes=(hash)
    hash.each do |key, value|
      send("#{key}=", value)
    end
  end

  def attributes
    { 'name' => nil }
  end
end

Now it is possible to create an instance of Person and set attributes using from_json.

irb> json = { name: 'Bob' }.to_json
irb> person = Person.new
irb> person.from_json(json)
=> #<Person:0x00000100c773f0 @name="Bob">
irb> person.name
=> "Bob"

1.10 Translation

ActiveModel::Translation provides integration between your object and the Rails internationalization (i18n) framework.

class Person
  extend ActiveModel::Translation
end

With the human_attribute_name method, you can transform attribute names into a more human-readable format. The human-readable format is defined in your locale file(s).

config/locales/app.pt-BR.yml

pt-BR:
  activemodel:
    attributes:
      person:
        name: 'Nome'

Person.human_attribute_name('name') # => "Nome"

1.11 Lint Tests

ActiveModel::Lint::Tests allows you to test whether an object is compliant with the Active Model API.

app/models/person.rb

class Person
  include ActiveModel::Model
end

test/models/person_test.rb

require "test_helper"

class PersonTest < ActiveSupport::TestCase
  include ActiveModel::Lint::Tests

  setup do
    @model = Person.new
  end
end

$ bin/rails test

Run options: --seed 14596

# Running:

......

Finished in 0.024899s, 240.9735 runs/s, 1204.8677 assertions/s.

6 runs, 30 assertions, 0 failures, 0 errors, 0 skips

An object is not required to implement all APIs in order to work with Action Pack. This module only intends to guide in case you want all features out of the box.

1.12 SecurePassword

ActiveModel::SecurePassword provides a way to securely store any password in an encrypted form. When you include this module, a has_secure_password class method is provided which defines a password accessor with certain validations on it by default.

1.12.1 Requirements

ActiveModel::SecurePassword depends on bcrypt, so include this gem in your Gemfile to use ActiveModel::SecurePassword correctly. In order to make this work, the model must have an accessor named XXX_digest. Where XXX is the attribute name of your desired password. The following validations are added automatically:

Password should be present.
Password should be equal to its confirmation (provided XXX_confirmation is passed along).
The maximum length of a password is 72 bytes (required as bcrypt, on which ActiveModel::SecurePassword depends, truncates the string to this size before encrypting it).

1.12.2 Examples

class Person
  include ActiveModel::SecurePassword
  has_secure_password
  has_secure_password :recovery_password, validations: false

  attr_accessor :password_digest, :recovery_password_digest
end

irb> person = Person.new

# When password is blank.
irb> person.valid?
=> false

# When the confirmation doesn't match the password.
irb> person.password = 'aditya'
irb> person.password_confirmation = 'nomatch'
irb> person.valid?
=> false

# When the length of password exceeds 72.
irb> person.password = person.password_confirmation = 'a' * 100
irb> person.valid?
=> false

# When only password is supplied with no password_confirmation.
irb> person.password = 'aditya'
irb> person.valid?
=> true

# When all validations are passed.
irb> person.password = person.password_confirmation = 'aditya'
irb> person.valid?
=> true

irb> person.recovery_password = "42password"

irb> person.authenticate('aditya')
=> #<Person> # == person
irb> person.authenticate('notright')
=> false
irb> person.authenticate_password('aditya')
=> #<Person> # == person
irb> person.authenticate_password('notright')
=> false

irb> person.authenticate_recovery_password('42password')
=> #<Person> # == person
irb> person.authenticate_recovery_password('notright')
=> false

irb> person.password_digest
=> "$2a$04$gF8RfZdoXHvyTjHhiU4ZsO.kQqV9oonYZu31PRE4hLQn3xM2qkpIy"
irb> person.recovery_password_digest
=> "$2a$04$iOfhwahFymCs5weB3BNH/uXkTG65HR.qpW.bNhEjFP3ftli3o5DQC"

Feedback

You're encouraged to help improve the quality of this guide.

Please contribute if you see any typos or factual errors. To get started, you can read our documentation contributions section.

You may also find incomplete content or stuff that is not up to date. Please do add any missing documentation for main. Make sure to check Edge Guides first to verify if the issues are already fixed or not on the main branch. Check the Ruby on Rails Guides Guidelines for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome on the official Ruby on Rails Forum.

Chapters