ActiveRecord::NamedScope::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').

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.

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

     # File lib/active_record/named_scope.rb, line 100
100:       def scope(name, scope_options = {}, &block)
101:         name = name.to_sym
102:         valid_scope_name?(name)
103: 
104:         extension = Module.new(&block) if block_given?
105: 
106:         scopes[name] = lambda do |*args|
107:           options = scope_options.is_a?(Proc) ? scope_options.call(*args) : scope_options
108: 
109:           relation = if options.is_a?(Hash)
110:             scoped.apply_finder_options(options)
111:           elsif options
112:             scoped.merge(options)
113:           else
114:             scoped
115:           end
116: 
117:           extension ? relation.extending(extension) : relation
118:         end
119: 
120:         singleton_class.send(:redefine_method, name, &scopes[name])
121:       end

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(:colour => '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.

    # File lib/active_record/named_scope.rb, line 28
28:       def scoped(options = nil)
29:         if options
30:           scoped.apply_finder_options(options)
31:         else
32:           current_scoped_methods ? relation.merge(current_scoped_methods) : relation.clone
33:         end
34:       end