scope

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.now - 1.week)
end

The example above would be ‘frozen’ to the Time.now 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.now - 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

# File activerecord/lib/active_record/named_scope.rb, line 173
      def scope(name, scope_options = {})
        name = name.to_sym
        valid_scope_name?(name)
        extension = Module.new(&Proc.new) if block_given?

        scope_proc = lambda do |*args|
          options = scope_options.respond_to?(:call) ? scope_options.call(*args) : scope_options
          options = scoped.apply_finder_options(options) if options.is_a?(Hash)

          relation = scoped.merge(options)

          extension ? relation.extending(extension) : relation
        end

        singleton_class.send(:redefine_method, name, &scope_proc)
      end