Adds a class method for retrieving and querying objects. A scope represents a narrowing of a database query, such as :conditions => {:color => :red}, :select => ‘shirts.*’, :include => :washing_instructions.

  class Shirt < ActiveRecord::Base
    named_scope :red, :conditions => {:color => 'red'}
    named_scope :dry_clean_only, :joins => :washing_instructions, :conditions => ['washing_instructions.dry_clean_only = ?', true]
  end

The above calls to named_scope define class methods Shirt.red and Shirt.dry_clean_only. Shirt.red, in effect, represents the query Shirt.find(:all, :conditions => {:color => ‘red’}).

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.find(:first), Shirt.red.count, Shirt.red.find(:all, :conditions => {: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
    named_scope :colored, lambda { |color|
      { :conditions => { :color => color } }
    }
  end

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

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

  class Shirt < ActiveRecord::Base
    named_scope :red, :conditions => {:color => 'red'} do
      def dom_id
        'red_shirts'
      end
    end
  end

For testing complex named \scopes, you can examine the scoping options using the proxy_options method on the proxy itself.

  class Shirt < ActiveRecord::Base
    named_scope :colored, lambda { |color|
      { :conditions => { :color => color } }
    }
  end

  expected_options = { :conditions => { :colored => 'red' } }
  assert_equal expected_options, Shirt.colored('red').proxy_options