Methods
Public Instance methods
named_scope(name, options = {}, &block)

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
     # File activerecord/lib/active_record/named_scope.rb, line 86
 86:       def named_scope(name, options = {}, &block)
 87:         name = name.to_sym
 88: 
 89:         scopes[name] = lambda do |parent_scope, *args|
 90:           Scope.new(parent_scope, case options
 91:             when Hash
 92:               options
 93:             when Proc
 94:               if self.model_name != parent_scope.model_name
 95:                 options.bind(parent_scope).call(*args)
 96:               else
 97:                 options.call(*args)
 98:               end
 99:           end, &block)
100:         end
101: 
102:         singleton_class.send :define_method, name do |*args|
103:           scopes[name].call(self, *args)
104:         end
105:       end
scoped(scope, &block)
    # File activerecord/lib/active_record/named_scope.rb, line 19
19:       def scoped(scope, &block)
20:         Scope.new(self, scope, &block)
21:       end
scopes()
    # File activerecord/lib/active_record/named_scope.rb, line 15
15:       def scopes
16:         read_inheritable_attribute(:scopes) || write_inheritable_attribute(:scopes, {})
17:       end