This Guide is based on Rails 3.0. Some of the code shown here will not work in other versions of Rails.
If you’re used to using raw SQL to find database records, then you will generally find that there are better ways to carry out the same operations in Rails. Active Record insulates you from the need to use SQL in most cases.
Code examples throughout this guide will refer to one or more of the following models:
All of the following models use id as the primary key, unless specified otherwise.
class Client < ActiveRecord::Base has_one :address has_many :orders has_and_belongs_to_many :roles end
class Address < ActiveRecord::Base belongs_to :client end
class Order < ActiveRecord::Base belongs_to :client, :counter_cache => true end
class Role < ActiveRecord::Base has_and_belongs_to_many :clients end
Active Record will perform queries on the database for you and is compatible with most database systems (MySQL, PostgreSQL and SQLite to name a few). Regardless of which database system you’re using, the Active Record method format will always be the same.
1 Retrieving Objects from the Database
To retrieve objects from the database, Active Record provides several finder methods. Each finder method allows you to pass arguments into it to perform certain queries on your database without writing raw SQL.
The methods are:
- where
- select
- group
- order
- reorder
- reverse_order
- limit
- offset
- joins
- includes
- lock
- readonly
- from
- having
All of the above methods return an instance of ActiveRecord::Relation.
The primary operation of Model.find(options) can be summarized as:
- Convert the supplied options to an equivalent SQL query.
- Fire the SQL query and retrieve the corresponding results from the database.
- Instantiate the equivalent Ruby object of the appropriate model for every resulting row.
- Run after_find callbacks, if any.
1.1 Retrieving a Single Object
Active Record provides five different ways of retrieving a single object.
1.1.1 Using a Primary Key
Using Model.find(primary_key), you can retrieve the object corresponding to the specified primary key that matches any supplied options. For example:
# Find the client with primary key (id) 10. client = Client.find(10) # => #<Client id: 10, first_name: "Ryan">
The SQL equivalent of the above is:
SELECT * FROM clients WHERE (clients.id = 10) LIMIT 1
Model.find(primary_key) will raise an ActiveRecord::RecordNotFound exception if no matching record is found.
1.1.2 first
Model.first finds the first record matched by the supplied options, if any. For example:
client = Client.first # => #<Client id: 1, first_name: "Lifo">
The SQL equivalent of the above is:
SELECT * FROM clients LIMIT 1
Model.first returns nil if no matching record is found. No exception will be raised.
1.1.3 last
Model.last finds the last record matched by the supplied options. For example:
client = Client.last # => #<Client id: 221, first_name: "Russel">
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1
Model.last returns nil if no matching record is found. No exception will be raised.
1.1.4 first!
Model.first! finds the first record. For example:
client = Client.first! # => #<Client id: 1, first_name: "Lifo">
The SQL equivalent of the above is:
SELECT * FROM clients LIMIT 1
Model.first! raises RecordNotFound if no matching record is found.
1.1.5 last!
Model.last! finds the last record. For example:
client = Client.last! # => #<Client id: 221, first_name: "Russel">
The SQL equivalent of the above is:
SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1
Model.last! raises RecordNotFound if no matching record is found.
1.2 Retrieving Multiple Objects
1.2.1 Using Multiple Primary Keys
Model.find(array_of_primary_key) accepts an array of primary keys, returning an array containing all of the matching records for the supplied primary keys. For example:
# Find the clients with primary keys 1 and 10. client = Client.find([1, 10]) # Or even Client.find(1, 10) # => [#<Client id: 1, first_name: "Lifo">, #<Client id: 10, first_name: "Ryan">]
The SQL equivalent of the above is:
SELECT * FROM clients WHERE (clients.id IN (1,10))
Model.find(array_of_primary_key) will raise an ActiveRecord::RecordNotFound exception unless a matching record is found for all of the supplied primary keys.
1.3 Retrieving Multiple Objects in Batches
We often need to iterate over a large set of records, as when we send a newsletter to a large set of users, or when we export data.
This may appear straightforward:
# This is very inefficient when the users table has thousands of rows. User.all.each do |user| NewsLetter.weekly_deliver(user) end
But this approach becomes increasingly impractical as the table size increases, since User.all.each instructs Active Record to fetch the entire table in a single pass, build a model object per row, and then keep the entire array of model objects in memory. Indeed, if we have a large number of records, the entire collection may exceed the amount of memory available.
Rails provides two methods that address this problem by dividing records into memory-friendly batches for processing. The first method, find_each, retrieves a batch of records and then yields each record to the block individually as a model. The second method, find_in_batches, retrieves a batch of records and then yields the entire batch to the block as an array of models.
The find_each and find_in_batches methods are intended for use in the batch processing of a large number of records that wouldn’t fit in memory all at once. If you just need to loop over a thousand records the regular find methods are the preferred option.
1.3.1 find_each
The find_each method retrieves a batch of records and then yields each record to the block individually as a model. In the following example, find_each will retrieve 1000 records (the current default for both find_each and find_in_batches) and then yield each record individually to the block as a model. This process is repeated until all of the records have been processed:
User.find_each do |user| NewsLetter.weekly_deliver(user) end
1.3.1.1 Options for find_each
The find_each method accepts most of the options allowed by the regular find method, except for :order and :limit, which are reserved for internal use by find_each.
Two additional options, :batch_size and :start, are available as well.
:batch_size
The :batch_size option allows you to specify the number of records to be retrieved in each batch, before being passed individually to the block. For example, to retrieve records in batches of 5000:
User.find_each(:batch_size => 5000) do |user| NewsLetter.weekly_deliver(user) end
:start
By default, records are fetched in ascending order of the primary key, which must be an integer. The :start option allows you to configure the first ID of the sequence whenever the lowest ID is not the one you need. This would be useful, for example, if you wanted to resume an interrupted batch process, provided you saved the last processed ID as a checkpoint.
For example, to send newsletters only to users with the primary key starting from 2000, and to retrieve them in batches of 5000:
User.find_each(:start => 2000, :batch_size => 5000) do |user| NewsLetter.weekly_deliver(user) end
Another example would be if you wanted multiple workers handling the same processing queue. You could have each worker handle 10000 records by setting the appropriate :start option on each worker.
The :include option allows you to name associations that should be loaded alongside with the models.
1.3.2 find_in_batches
The find_in_batches method is similar to find_each, since both retrieve batches of records. The difference is that find_in_batches yields batches to the block as an array of models, instead of individually. The following example will yield to the supplied block an array of up to 1000 invoices at a time, with the final block containing any remaining invoices:
# Give add_invoices an array of 1000 invoices at a time Invoice.find_in_batches(:include => :invoice_lines) do |invoices| export.add_invoices(invoices) end
The :include option allows you to name associations that should be loaded alongside with the models.
1.3.2.1 Options for find_in_batches
The find_in_batches method accepts the same :batch_size and :start options as find_each, as well as most of the options allowed by the regular find method, except for :order and :limit, which are reserved for internal use by find_in_batches.
2 Conditions
The where method allows you to specify conditions to limit the records returned, representing the WHERE-part of the SQL statement. Conditions can either be specified as a string, array, or hash.
2.1 Pure String Conditions
If you’d like to add conditions to your find, you could just specify them in there, just like Client.where("orders_count = '2'"). This will find all clients where the orders_count field’s value is 2.
Building your own conditions as pure strings can leave you vulnerable to SQL injection exploits. For example, Client.where("first_name LIKE '%#{params[:first_name]}%'") is not safe. See the next section for the preferred way to handle conditions using an array.
2.2 Array Conditions
Now what if that number could vary, say as an argument from somewhere? The find would then take the form:
Client.where("orders_count = ?", params[:orders])
Active Record will go through the first element in the conditions value and any additional elements will replace the question marks (?) in the first element.
If you want to specify multiple conditions:
Client.where("orders_count = ? AND locked = ?", params[:orders], false)
In this example, the first question mark will be replaced with the value in params[:orders] and the second will be replaced with the SQL representation of false, which depends on the adapter.
This code is highly preferable:
Client.where("orders_count = ?", params[:orders])
to this code:
Client.where("orders_count = #{params[:orders]}")
because of argument safety. Putting the variable directly into the conditions string will pass the variable to the database as-is. This means that it will be an unescaped variable directly from a user who may have malicious intent. If you do this, you put your entire database at risk because once a user finds out he or she can exploit your database they can do just about anything to it. Never ever put your arguments directly inside the conditions string.
For more information on the dangers of SQL injection, see the Ruby on Rails Security Guide.
2.2.1 Placeholder Conditions
Similar to the (?) replacement style of params, you can also specify keys/values hash in your array conditions:
Client.where("created_at >= :start_date AND created_at <= :end_date", {:start_date => params[:start_date], :end_date => params[:end_date]})
This makes for clearer readability if you have a large number of variable conditions.
2.2.2 Range Conditions
If you’re looking for a range inside of a table (for example, users created in a certain timeframe) you can use the conditions option coupled with the IN SQL statement for this. If you had two dates coming in from a controller you could do something like this to look for a range:
Client.where(:created_at => (params[:start_date].to_date)..(params[:end_date].to_date))
This query will generate something similar to the following SQL:
SELECT "clients".* FROM "clients" WHERE ("clients"."created_at" BETWEEN '2010-09-29' AND '2010-11-30')
2.3 Hash Conditions
Active Record also allows you to pass in hash conditions which can increase the readability of your conditions syntax. With hash conditions, you pass in a hash with keys of the fields you want conditionalised and the values of how you want to conditionalise them:
Only equality, range and subset checking are possible with Hash conditions.
2.3.1 Equality Conditions
Client.where(:locked => true)
The field name can also be a string:
Client.where('locked' => true)
2.3.2 Range Conditions
The good thing about this is that we can pass in a range for our fields without it generating a large query as shown in the preamble of this section.
Client.where(:created_at => (Time.now.midnight - 1.day)..Time.now.midnight)
This will find all clients created yesterday by using a BETWEEN SQL statement:
SELECT * FROM clients WHERE (clients.created_at BETWEEN '2008-12-21 00:00:00' AND '2008-12-22 00:00:00')
This demonstrates a shorter syntax for the examples in Array Conditions
2.3.3 Subset Conditions
If you want to find records using the IN expression you can pass an array to the conditions hash:
Client.where(:orders_count => [1,3,5])
This code will generate SQL like this:
SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))
3 Ordering
To retrieve records from the database in a specific order, you can use the order method.
For example, if you’re getting a set of records and want to order them in ascending order by the created_at field in your table:
Client.order("created_at")
You could specify ASC or DESC as well:
Client.order("created_at DESC") # OR Client.order("created_at ASC")
Or ordering by multiple fields:
Client.order("orders_count ASC, created_at DESC")
4 Selecting Specific Fields
By default, Model.find selects all the fields from the result set using select *.
To select only a subset of fields from the result set, you can specify the subset via the select method.
If the select method is used, all the returning objects will be read only.
For example, to select only viewable_by and locked columns:
Client.select("viewable_by, locked")
The SQL query used by this find call will be somewhat like:
SELECT viewable_by, locked FROM clients
Be careful because this also means you’re initializing a model object with only the fields that you’ve selected. If you attempt to access a field that is not in the initialized record you’ll receive:
ActiveModel::MissingAttributeError: missing attribute: <attribute>
Where <attribute> is the attribute you asked for. The id method will not raise the ActiveRecord::MissingAttributeError, so just be careful when working with associations because they need the id method to function properly.
If you would like to only grab a single record per unique value in a certain field, you can use uniq:
Client.select(:name).uniq
This would generate SQL like:
SELECT DISTINCT name FROM clients
You can also remove the uniqueness constraint:
query = Client.select(:name).uniq # => Returns unique names query.uniq(false) # => Returns all names, even if there are duplicates
5 Limit and Offset
To apply LIMIT to the SQL fired by the Model.find, you can specify the LIMIT using limit and offset methods on the relation.
You can use limit to specify the number of records to be retrieved, and use offset to specify the number of records to skip before starting to return the records. For example
Client.limit(5)
will return a maximum of 5 clients and because it specifies no offset it will return the first 5 in the table. The SQL it executes looks like this:
SELECT * FROM clients LIMIT 5
Adding offset to that
Client.limit(5).offset(30)
will return instead a maximum of 5 clients beginning with the 31st. The SQL looks like:
SELECT * FROM clients LIMIT 5 OFFSET 30
6 Group
To apply a GROUP BY clause to the SQL fired by the finder, you can specify the group method on the find.
For example, if you want to find a collection of the dates orders were created on:
Order.select("date(created_at) as ordered_date, sum(price) as total_price").group("date(created_at)")
And this will give you a single Order object for each date where there are orders in the database.
The SQL that would be executed would be something like this:
SELECT date(created_at) as ordered_date, sum(price) as total_price FROM orders GROUP BY date(created_at)
7 Having
SQL uses the HAVING clause to specify conditions on the GROUP BY fields. You can add the HAVING clause to the SQL fired by the Model.find by adding the :having option to the find.
For example:
Order.select("date(created_at) as ordered_date, sum(price) as total_price").group("date(created_at)").having("sum(price) > ?", 100)
The SQL that would be executed would be something like this:
SELECT date(created_at) as ordered_date, sum(price) as total_price FROM orders GROUP BY date(created_at) HAVING sum(price) > 100
This will return single order objects for each day, but only those that are ordered more than $100 in a day.
8 Overriding Conditions
8.1 except
You can specify certain conditions to be excepted by using the except method. For example:
Post.where('id > 10').limit(20).order('id asc').except(:order)
The SQL that would be executed:
SELECT * FROM posts WHERE id > 10 LIMIT 20
8.2 only
You can also override conditions using the only method. For example:
Post.where('id > 10').limit(20).order('id desc').only(:order, :where)
The SQL that would be executed:
SELECT * FROM posts WHERE id > 10 ORDER BY id DESC
8.3 reorder
The reorder method overrides the default scope order. For example:
class Post < ActiveRecord::Base .. .. has_many :comments, :order => 'posted_at DESC' end Post.find(10).comments.reorder('name')
The SQL that would be executed:
SELECT * FROM posts WHERE id = 10 ORDER BY name
In case the reorder clause is not used, the SQL executed would be:
SELECT * FROM posts WHERE id = 10 ORDER BY posted_at DESC
8.4 reverse_order
The reverse_order method reverses the ordering clause if specified.
Client.where("orders_count > 10").order(:name).reverse_order
The SQL that would be executed:
SELECT * FROM clients WHERE orders_count > 10 ORDER BY name DESC
If no ordering clause is specified in the query, the reverse_order orders by the primary key in reverse order.
Client.where("orders_count > 10").reverse_order
The SQL that would be executed:
SELECT * FROM clients WHERE orders_count > 10 ORDER BY clients.id DESC
This method accepts no arguments.
9 Readonly Objects
Active Record provides readonly method on a relation to explicitly disallow modification or deletion of any of the returned object. Any attempt to alter or destroy a readonly record will not succeed, raising an ActiveRecord::ReadOnlyRecord exception.
client = Client.readonly.first client.visits += 1 client.save
As client is explicitly set to be a readonly object, the above code will raise an ActiveRecord::ReadOnlyRecord exception when calling client.save with an updated value of visits.
10 Locking Records for Update
Locking is helpful for preventing race conditions when updating records in the database and ensuring atomic updates.
Active Record provides two locking mechanisms:
- Optimistic Locking
- Pessimistic Locking
10.1 Optimistic Locking
Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of conflicts with the data. It does this by checking whether another process has made changes to a record since it was opened. An ActiveRecord::StaleObjectError exception is thrown if that has occurred and the update is ignored.
Optimistic locking column
In order to use optimistic locking, the table needs to have a column called lock_version. Each time the record is updated, Active Record increments the lock_version column. If an update request is made with a lower value in the lock_version field than is currently in the lock_version column in the database, the update request will fail with an ActiveRecord::StaleObjectError. Example:
c1 = Client.find(1) c2 = Client.find(1) c1.first_name = "Michael" c1.save c2.name = "should fail" c2.save # Raises an ActiveRecord::StaleObjectError
You’re then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging, or otherwise apply the business logic needed to resolve the conflict.
You must ensure that your database schema defaults the lock_version column to 0.
This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false.
To override the name of the lock_version column, ActiveRecord::Base provides a class method called set_locking_column:
class Client < ActiveRecord::Base set_locking_column :lock_client_column end
10.2 Pessimistic Locking
Pessimistic locking uses a locking mechanism provided by the underlying database. Using lock when building a relation obtains an exclusive lock on the selected rows. Relations using lock are usually wrapped inside a transaction for preventing deadlock conditions.
For example:
Item.transaction do i = Item.lock.first i.name = 'Jones' i.save end
The above session produces the following SQL for a MySQL backend:
SQL (0.2ms) BEGIN Item Load (0.3ms) SELECT * FROM `items` LIMIT 1 FOR UPDATE Item Update (0.4ms) UPDATE `items` SET `updated_at` = '2009-02-07 18:05:56', `name` = 'Jones' WHERE `id` = 1 SQL (0.8ms) COMMIT
You can also pass raw SQL to the lock method for allowing different types of locks. For example, MySQL has an expression called LOCK IN SHARE MODE where you can lock a record but still allow other queries to read it. To specify this expression just pass it in as the lock option:
Item.transaction do i = Item.lock("LOCK IN SHARE MODE").find(1) i.increment!(:views) end
If you already have an instance of your model, you can start a transaction and acquire the lock in one go using the following code:
item = Item.first item.with_lock do # This block is called within a transaction, # item is already locked. item.increment!(:views) end
11 Joining Tables
Active Record provides a finder method called joins for specifying JOIN clauses on the resulting SQL. There are multiple ways to use the joins method.
11.1 Using a String SQL Fragment
You can just supply the raw SQL specifying the JOIN clause to joins:
Client.joins('LEFT OUTER JOIN addresses ON addresses.client_id = clients.id')
This will result in the following SQL:
SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id
11.2 Using Array/Hash of Named Associations
This method only works with INNER JOIN.
Active Record lets you use the names of the associations defined on the model as a shortcut for specifying JOIN clause for those associations when using the joins method.
For example, consider the following Category, Post, Comments and Guest models:
class Category < ActiveRecord::Base has_many :posts end class Post < ActiveRecord::Base belongs_to :category has_many :comments has_many :tags end class Comment < ActiveRecord::Base belongs_to :post has_one :guest end class Guest < ActiveRecord::Base belongs_to :comment end class Tag < ActiveRecord::Base belongs_to :post end
Now all of the following will produce the expected join queries using INNER JOIN:
11.2.1 Joining a Single Association
Category.joins(:posts)
This produces:
SELECT categories.* FROM categories INNER JOIN posts ON posts.category_id = categories.id
Or, in English: “return a Category object for all categories with posts”. Note that you will see duplicate categories if more than one post has the same category. If you want unique categories, you can use Category.joins(:post).select(“distinct(categories.id)”).
11.2.2 Joining Multiple Associations
Post.joins(:category, :comments)
This produces:
SELECT posts.* FROM posts INNER JOIN categories ON posts.category_id = categories.id INNER JOIN comments ON comments.post_id = posts.id
Or, in English: “return all posts that have a category and at least one comment”. Note again that posts with multiple comments will show up multiple times.
11.2.3 Joining Nested Associations (Single Level)
Post.joins(:comments => :guest)
This produces:
SELECT posts.* FROM posts INNER JOIN comments ON comments.post_id = posts.id INNER JOIN guests ON guests.comment_id = comments.id
Or, in English: “return all posts that have a comment made by a guest.”
11.2.4 Joining Nested Associations (Multiple Level)
Category.joins(:posts => [{:comments => :guest}, :tags])
This produces:
SELECT categories.* FROM categories INNER JOIN posts ON posts.category_id = categories.id INNER JOIN comments ON comments.post_id = posts.id INNER JOIN guests ON guests.comment_id = comments.id INNER JOIN tags ON tags.post_id = posts.id
11.3 Specifying Conditions on the Joined Tables
You can specify conditions on the joined tables using the regular Array and String conditions. Hash conditions provides a special syntax for specifying conditions for the joined tables:
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.joins(:orders).where('orders.created_at' => time_range)
An alternative and cleaner syntax is to nest the hash conditions:
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.joins(:orders).where(:orders => {:created_at => time_range})
This will find all clients who have orders that were created yesterday, again using a BETWEEN SQL expression.
12 Eager Loading Associations
Eager loading is the mechanism for loading the associated records of the objects returned by Model.find using as few queries as possible.
N + 1 queries problem
Consider the following code, which finds 10 clients and prints their postcodes:
clients = Client.limit(10) clients.each do |client| puts client.address.postcode end
This code looks fine at the first sight. But the problem lies within the total number of queries executed. The above code executes 1 ( to find 10 clients ) + 10 ( one per each client to load the address ) = 11 queries in total.
Solution to N + 1 queries problem
Active Record lets you specify in advance all the associations that are going to be loaded. This is possible by specifying the includes method of the Model.find call. With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.
Revisiting the above case, we could rewrite Client.all to use eager load addresses:
clients = Client.includes(:address).limit(10) clients.each do |client| puts client.address.postcode end
The above code will execute just 2 queries, as opposed to 11 queries in the previous case:
SELECT * FROM clients LIMIT 10 SELECT addresses.* FROM addresses WHERE (addresses.client_id IN (1,2,3,4,5,6,7,8,9,10))
12.1 Eager Loading Multiple Associations
Active Record lets you eager load any number of associations with a single Model.find call by using an array, hash, or a nested hash of array/hash with the includes method.
12.1.1 Array of Multiple Associations
Post.includes(:category, :comments)
This loads all the posts and the associated category and comments for each post.
12.1.2 Nested Associations Hash
Category.includes(:posts => [{:comments => :guest}, :tags]).find(1)
This will find the category with id 1 and eager load all of the associated posts, the associated posts’ tags and comments, and every comment’s guest association.
12.2 Specifying Conditions on Eager Loaded Associations
Even though Active Record lets you specify conditions on the eager loaded associations just like joins, the recommended way is to use joins instead.
However if you must do this, you may use where as you would normally.
Post.includes(:comments).where("comments.visible", true)
This would generate a query which contains a LEFT OUTER JOIN whereas the joins method would generate one using the INNER JOIN function instead.
SELECT "posts"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5 FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" WHERE (comments.visible = 1)
If there was no where condition, this would generate the normal set of two queries.
If, in the case of this includes query, there were no comments for any posts, all the posts would still be loaded. By using joins (an INNER JOIN), the join conditions must match, otherwise no records will be returned.
13 Scopes
Scoping allows you to specify commonly-used ARel queries which can be referenced as method calls on the association objects or models. With these scopes, you can use every method previously covered such as where, joins and includes. All scope methods will return an ActiveRecord::Relation object which will allow for further methods (such as other scopes) to be called on it.
To define a simple scope, we use the scope method inside the class, passing the ARel query that we’d like run when this scope is called:
class Post < ActiveRecord::Base scope :published, where(:published => true) end
Just like before, these methods are also chainable:
class Post < ActiveRecord::Base scope :published, where(:published => true).joins(:category) end
Scopes are also chainable within scopes:
class Post < ActiveRecord::Base scope :published, where(:published => true) scope :published_and_commented, published.and(self.arel_table[:comments_count].gt(0)) end
To call this published scope we can call it on either the class:
Post.published # => [published posts]
Or on an association consisting of Post objects:
category = Category.first category.posts.published # => [published posts belonging to this category]
13.1 Working with times
If you’re working with dates or times within scopes, due to how they are evaluated, you will need to use a lambda so that the scope is evaluated every time.
class Post < ActiveRecord::Base scope :last_week, lambda { where("created_at < ?", Time.zone.now ) } end
Without the lambda, this Time.zone.now will only be called once.
13.2 Passing in arguments
When a lambda is used for a scope, it can take arguments:
class Post < ActiveRecord::Base scope :1_week_before, lambda { |time| where("created_at < ?", time) } end
This may then be called using this:
Post.1_week_before(Time.zone.now)
However, this is just duplicating the functionality that would be provided to you by a class method.
class Post < ActiveRecord::Base def self.1_week_before(time) where("created_at < ?", time) end end
Using a class method is the preferred way to accept arguments for scopes. These methods will still be accessible on the association objects:
category.posts.1_week_before(time)
13.3 Working with scopes
Where a relational object is required, the scoped method may come in handy. This will return an ActiveRecord::Relation object which can have further scoping applied to it afterwards. A place where this may come in handy is on associations
client = Client.find_by_first_name("Ryan") orders = client.orders.scoped
With this new orders object, we are able to ascertain that this object can have more scopes applied to it. For instance, if we wanted to return orders only in the last 30 days at a later point.
orders.where("created_at > ?", 30.days.ago)
13.4 Applying a default scope
If we wish for a scope to be applied across all queries to the model we can use the default_scope method within the model itself.
class Client < ActiveRecord::Base default_scope where("removed_at IS NULL") end
When queries are executed on this model, the SQL query will now look something like this:
SELECT * FROM clients WHERE removed_at IS NULL
13.5 Removing all scoping
If we wish to remove scoping for any reason we can use the unscoped method. This is especially useful if a default_scope is specified in the model and should not be applied for this particular query.
Client.unscoped.all
This method removes all scoping and will do a normal query on the table.
14 Dynamic Finders
For every field (also known as an attribute) you define in your table, Active Record provides a finder method. If you have a field called first_name on your Client model for example, you get find_by_first_name and find_all_by_first_name for free from Active Record. If you have a locked field on the Client model, you also get find_by_locked and find_all_by_locked methods.
You can also use find_last_by_* methods which will find the last record matching your argument.
You can specify an exclamation point (!) on the end of the dynamic finders to get them to raise an ActiveRecord::RecordNotFound error if they do not return any records, like Client.find_by_name!("Ryan")
If you want to find both by name and locked, you can chain these finders together by simply typing “and” between the fields. For example, Client.find_by_first_name_and_locked("Ryan", true).
Up to and including Rails 3.1, when the number of arguments passed to a dynamic finder method is lesser than the number of fields, say Client.find_by_name_and_locked(“Ryan”), the behavior is to pass nil as the missing argument. This is unintentional and this behavior will be changed in Rails 3.2 to throw an ArgumentError.
15 Find or build a new object
It’s common that you need to find a record or create it if it doesn’t exist. You can do that with the first_or_create and first_or_create! methods.
15.1 first_or_create
The first_or_create method checks whether first returns nil or not. If it does return nil, then create is called. This is very powerful when coupled with the where method. Let’s see an example.
Suppose you want to find a client named ‘Andy’, and if there’s none, create one and additionally set his locked attribute to false. You can do so by running:
Client.where(:first_name => 'Andy').first_or_create(:locked => false) # => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: false, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">
The SQL generated by this method looks like this:
SELECT * FROM clients WHERE (clients.first_name = 'Andy') LIMIT 1 BEGIN INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES ('2011-08-30 05:22:57', 'Andy', 0, NULL, '2011-08-30 05:22:57') COMMIT
first_or_create returns either the record that already exists or the new record. In our case, we didn’t already have a client named Andy so the record is created and returned.
The new record might not be saved to the database; that depends on whether validations passed or not (just like create).
It’s also worth noting that first_or_create takes into account the arguments of the where method. In the example above we didn’t explicitly pass a :first_name => 'Andy' argument to first_or_create. However, that was used when creating the new record because it was already passed before to the where method.
You can do the same with the find_or_create_by method:
Client.find_or_create_by_first_name(:first_name => "Andy", :locked => false)
This method still works, but it’s encouraged to use first_or_create because it’s more explicit on which arguments are used to find the record and which are used to create, resulting in less confusion overall.
15.2 first_or_create!
You can also use first_or_create! to raise an exception if the new record is invalid. Validations are not covered on this guide, but let’s assume for a moment that you temporarily add
validates :orders_count, :presence => true
to your Client model. If you try to create a new Client without passing an orders_count, the record will be invalid and an exception will be raised:
Client.where(:first_name => 'Andy').first_or_create!(:locked => false) # => ActiveRecord::RecordInvalid: Validation failed: Orders count can't be blank
As with first_or_create there is a find_or_create_by! method but the first_or_create! method is preferred for clarity.
15.3 first_or_initialize
The first_or_initialize method will work just like first_or_create but it will not call create but new. This means that a new model instance will be created in memory but won’t be saved to the database. Continuing with the first_or_create example, we now want the client named ‘Nick’:
nick = Client.where(:first_name => 'Nick').first_or_initialize(:locked => false) # => <Client id: nil, first_name: "Nick", orders_count: 0, locked: false, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27"> nick.persisted? # => false nick.new_record? # => true
Because the object is not yet stored in the database, the SQL generated looks like this:
SELECT * FROM clients WHERE (clients.first_name = 'Nick') LIMIT 1
When you want to save it to the database, just call save:
nick.save # => true
16 Finding by SQL
If you’d like to use your own SQL to find records in a table you can use find_by_sql. The find_by_sql method will return an array of objects even if the underlying query returns just a single record. For example you could run this query:
Client.find_by_sql("SELECT * FROM clients INNER JOIN orders ON clients.id = orders.client_id ORDER clients.created_at desc")
find_by_sql provides you with a simple way of making custom calls to the database and retrieving instantiated objects.
17 select_all
find_by_sql has a close relative called connection#select_all. select_all will retrieve objects from the database using custom SQL just like find_by_sql but will not instantiate them. Instead, you will get an array of hashes where each hash indicates a record.
Client.connection.select_all("SELECT * FROM clients WHERE id = '1'")
18 pluck
pluck can be used to query a single column from the underlying table of a model. It accepts a column name as argument and returns an array of values of the specified column with the corresponding data type.
Client.where(:active => true).pluck(:id) # SELECT id FROM clients WHERE active = 1 Client.uniq.pluck(:role) # SELECT DISTINCT role FROM clients
pluck makes it possible to replace code like
Client.select(:id).map { |c| c.id }
with
Client.pluck(:id)
19 Existence of Objects
If you simply want to check for the existence of the object there’s a method called exists?. This method will query the database using the same query as find, but instead of returning an object or collection of objects it will return either true or false.
Client.exists?(1)
The exists? method also takes multiple ids, but the catch is that it will return true if any one of those records exists.
Client.exists?(1,2,3) # or Client.exists?([1,2,3])
It’s even possible to use exists? without any arguments on a model or a relation.
Client.where(:first_name => 'Ryan').exists?
The above returns true if there is at least one client with the first_name ‘Ryan’ and false otherwise.
Client.exists?
The above returns false if the clients table is empty and true otherwise.
You can also use any? and many? to check for existence on a model or relation.
# via a model Post.any? Post.many? # via a named scope Post.recent.any? Post.recent.many? # via a relation Post.where(:published => true).any? Post.where(:published => true).many? # via an association Post.first.categories.any? Post.first.categories.many?
20 Calculations
This section uses count as an example method in this preamble, but the options described apply to all sub-sections.
All calculation methods work directly on a model:
Client.count # SELECT count(*) AS count_all FROM clients
Or on a relation:
Client.where(:first_name => 'Ryan').count # SELECT count(*) AS count_all FROM clients WHERE (first_name = 'Ryan')
You can also use various finder methods on a relation for performing complex calculations:
Client.includes("orders").where(:first_name => 'Ryan', :orders => {:status => 'received'}).count
Which will execute:
SELECT count(DISTINCT clients.id) AS count_all FROM clients LEFT OUTER JOIN orders ON orders.client_id = client.id WHERE (clients.first_name = 'Ryan' AND orders.status = 'received')
20.1 Count
If you want to see how many records are in your model’s table you could call Client.count and that will return the number. If you want to be more specific and find all the clients with their age present in the database you can use Client.count(:age).
For options, please see the parent section, Calculations.
20.2 Average
If you want to see the average of a certain number in one of your tables you can call the average method on the class that relates to the table. This method call will look something like this:
Client.average("orders_count")
This will return a number (possibly a floating point number such as 3.14159265) representing the average value in the field.
For options, please see the parent section, Calculations.
20.3 Minimum
If you want to find the minimum value of a field in your table you can call the minimum method on the class that relates to the table. This method call will look something like this:
Client.minimum("age")
For options, please see the parent section, Calculations.
20.4 Maximum
If you want to find the maximum value of a field in your table you can call the maximum method on the class that relates to the table. This method call will look something like this:
Client.maximum("age")
For options, please see the parent section, Calculations.
20.5 Sum
If you want to find the sum of a field for all records in your table you can call the sum method on the class that relates to the table. This method call will look something like this:
Client.sum("orders_count")
For options, please see the parent section, Calculations.
21 Running EXPLAIN
You can run EXPLAIN on the queries triggered by relations. For example,
User.where(:id => 1).joins(:posts).explain
may yield
EXPLAIN for: SELECT `users`.* FROM `users` INNER JOIN `posts` ON `posts`.`user_id` = `users`.`id` WHERE `users`.`id` = 1 +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 1 | SIMPLE | posts | ALL | NULL | NULL | NULL | NULL | 1 | Using where | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+ 2 rows in set (0.00 sec)
under MySQL.
Active Record performs a pretty printing that emulates the one of the database shells. So, the same query running with the PostgreSQL adapter would yield instead
EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE "users"."id" = 1 QUERY PLAN ------------------------------------------------------------------------------ Nested Loop Left Join (cost=0.00..37.24 rows=8 width=0) Join Filter: (posts.user_id = users.id) -> Index Scan using users_pkey on users (cost=0.00..8.27 rows=1 width=4) Index Cond: (id = 1) -> Seq Scan on posts (cost=0.00..28.88 rows=8 width=4) Filter: (posts.user_id = 1) (6 rows)
Eager loading may trigger more than one query under the hood, and some queries may need the results of previous ones. Because of that, explain actually executes the query, and then asks for the query plans. For example,
User.where(:id => 1).includes(:posts).explain
yields
EXPLAIN for: SELECT `users`.* FROM `users` WHERE `users`.`id` = 1 +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ 1 row in set (0.00 sec) EXPLAIN for: SELECT `posts`.* FROM `posts` WHERE `posts`.`user_id` IN (1) +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | posts | ALL | NULL | NULL | NULL | NULL | 1 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
under MySQL.
21.1 Automatic EXPLAIN
Active Record is able to run EXPLAIN automatically on slow queries and log its output. This feature is controlled by the configuration parameter
config.active_record.auto_explain_threshold_in_seconds
If set to a number, any query exceeding those many seconds will have its EXPLAIN automatically triggered and logged. In the case of relations, the threshold is compared to the total time needed to fetch records. So, a relation is seen as a unit of work, no matter whether the implementation of eager loading involves several queries under the hood.
A threshold of nil disables automatic EXPLAINs.
The default threshold in development mode is 0.5 seconds, and nil in test and production modes.
Automatic EXPLAIN gets disabled if Active Record has no logger, regardless of the value of the threshold.
21.1.1 Disabling Automatic EXPLAIN
Automatic EXPLAIN can be selectively silenced with ActiveRecord::Base.silence_auto_explain:
ActiveRecord::Base.silence_auto_explain do # no automatic EXPLAIN is triggered here end
That may be useful for queries you know are slow but fine, like a heavyweight report of an admin interface.
As its name suggests, silence_auto_explain only silences automatic EXPLAINs. Explicit calls to ActiveRecord::Relation#explain run.
21.2 Interpreting EXPLAIN
Interpretation of the output of EXPLAIN is beyond the scope of this guide. The following pointers may be helpful:
- SQLite3: EXPLAIN QUERY PLAN
- MySQL: EXPLAIN Output Format
- PostgreSQL: Using EXPLAIN
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