# File activerecord/lib/active_record/scoping/named.rb, line 26
        def all
          scope = current_scope

          if scope
            if scope._deprecated_scope_source
              ActiveSupport::Deprecation.warn(<<~MSG.squish)
                Class level methods will no longer inherit scoping from `#{scope._deprecated_scope_source}`
                in Rails 6.1. To continue using the scoped relation, pass it into the block directly.
                To instead access the full set of models, as Rails 6.1 will, use `#{name}.unscoped`.
              MSG
            end

            if self == scope.klass
              scope.clone
            else
              relation.merge!(scope)
            end
          else
            default_scoped
          end
        end

Returns an ActiveRecord::Relation scope object.

posts = Post.all
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.all
fruits = fruits.where(color: 'red') if options[:red_only]
fruits = fruits.limit(10) if limited?

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

# File activerecord/lib/active_record/scoping/named.rb, line 169
def scope(name, body, &block)
  unless body.respond_to?(:call)
    raise ArgumentError, "The scope body needs to be callable."
  end

  if dangerous_class_method?(name)
    raise ArgumentError, "You tried to define a scope named \"#{name}\" " \
      "on the model \"#{self.name}\", but Active Record already defined " \
      "a class method with the same name."
  end

  if method_defined_within?(name, Relation)
    raise ArgumentError, "You tried to define a scope named \"#{name}\" " \
      "on the model \"#{self.name}\", but ActiveRecord::Relation already defined " \
      "an instance method with the same name."
  end

  valid_scope_name?(name)
  extension = Module.new(&block) if block

  if body.respond_to?(:to_proc)
    singleton_class.define_method(name) do |*args|
      scope = all._exec_scope(name, *args, &body)
      scope = scope.extending(extension) if extension
      scope
    end
  else
    singleton_class.define_method(name) do |*args|
      scope = body.call(*args) || all
      scope = scope.extending(extension) if extension
      scope
    end
  end

  generate_relation_method(name)
end

Adds a class method for retrieving and querying objects. The method is intended to return an ActiveRecord::Relation object, which is composable with other scopes. If it returns nil or false, an all scope is returned instead.

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').

You should always pass a callable object to the scopes defined with scope. This ensures that the scope is re-evaluated each time it is called.

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 but an ActiveRecord::Relation, which is composable with other scopes; 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 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
    pluck(:title)
  end
end

We are able to call the methods like this:

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