Module ActiveRecord::Scoping::Named::ClassMethods

Adds a class method for retrieving and querying objects. A scope represents a narrowing of a database query, such as where(:color => :red).select('shirts.*').includes(:washing_instructions).

  class Shirt < ActiveRecord::Base
    scope :red, where(:color => 'red')
    scope :dry_clean_only, joins(:washing_instructions).where('washing_instructions.dry_clean_only = ?', true)
  end

The above calls to scope define class methods Shirt.red and Shirt.dry_clean_only. Shirt.red, in effect, represents the query Shirt.where(:color => 'red').

Note that this is simply ‘syntactic sugar’ for defining an actual class method:

  class Shirt < ActiveRecord::Base
    def self.red
      where(:color => 'red')
    end
  end

Unlike Shirt.find(...), however, the object returned by Shirt.red is not an Array; it resembles the association object constructed by a has_many declaration. For instance, you can invoke Shirt.red.first, Shirt.red.count, Shirt.red.where(:size => 'small'). Also, just as with the association objects, named scopes act like an Array, implementing Enumerable; Shirt.red.each(&block), Shirt.red.first, and Shirt.red.inject(memo, &block) all behave as if Shirt.red really was an Array.

These named scopes are composable. For instance, Shirt.red.dry_clean_only will produce all shirts that are both red and dry clean only. Nested finds and calculations also work with these compositions: Shirt.red.dry_clean_only.count returns the number of garments for which these criteria obtain. Similarly with Shirt.red.dry_clean_only.average(:thread_count).

All scopes are available as class methods on the ActiveRecord::Base descendant upon which the scopes were defined. But they are also available to has_many associations. If,

  class Person < ActiveRecord::Base
    has_many :shirts
  end

then elton.shirts.red.dry_clean_only will return all of Elton’s red, dry clean only shirts.

Named scopes can also be procedural:

  class Shirt < ActiveRecord::Base
    scope :colored, lambda { |color| where(:color => color) }
  end

In this example, Shirt.colored('puce') finds all puce shirts.

On Ruby 1.9 you can use the ‘stabby lambda’ syntax:

  scope :colored, ->(color) { where(:color => color) }

Note that scopes defined with scope will be evaluated when they are defined, rather than when they are used. For example, the following would be incorrect:

  class Post < ActiveRecord::Base
    scope :recent, where('published_at >= ?', Time.current - 1.week)
  end

The example above would be ‘frozen’ to the Time.current value when the Post class was defined, and so the resultant SQL query would always be the same. The correct way to do this would be via a lambda, which will re-evaluate the scope each time it is called:

  class Post < ActiveRecord::Base
    scope :recent, lambda { where('published_at >= ?', Time.current - 1.week) }
  end

Named scopes can also have extensions, just as with has_many declarations:

  class Shirt < ActiveRecord::Base
    scope :red, where(:color => 'red') do
      def dom_id
        'red_shirts'
      end
    end
  end

Scopes can also be used while creating/building a record.

  class Article < ActiveRecord::Base
    scope :published, where(:published => true)
  end

  Article.published.new.published    # => true
  Article.published.create.published # => true

Class methods on your model are automatically available on scopes. Assuming the following setup:

  class Article < ActiveRecord::Base
    scope :published, where(:published => true)
    scope :featured, where(:featured => true)

    def self.latest_article
      order('published_at desc').first
    end

    def self.titles
      map(&:title)
    end

  end

We are able to call the methods like this:

  Article.published.featured.latest_article
  Article.featured.titles

Returns an anonymous scope.

  posts = Post.scoped
  posts.size # Fires "select count(*) from  posts" and returns the count
  posts.each {|p| puts p.name } # Fires "select * from posts" and loads post objects

  fruits = Fruit.scoped
  fruits = fruits.where(:color => 'red') if options[:red_only]
  fruits = fruits.limit(10) if limited?

Anonymous scopes tend to be useful when procedurally generating complex queries, where passing intermediate values (scopes) around as first-class objects is convenient.

You can define a scope that applies to all finders using ActiveRecord::Base.default_scope.