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Active Support Concern

A typical module looks like this:

module M
  def self.included(base)
    base.extend ClassMethods
    base.class_eval do
      scope :disabled, -> { where(disabled: true) }
    end
  end

  module ClassMethods
    ...
  end
end

By using ActiveSupport::Concern the above module could instead be written as:

require "active_support/concern"

module M
  extend ActiveSupport::Concern

  included do
    scope :disabled, -> { where(disabled: true) }
  end

  class_methods do
    ...
  end
end

Moreover, it gracefully handles module dependencies. Given a Foo module and a Bar module which depends on the former, we would typically write the following:

module Foo
  def self.included(base)
    base.class_eval do
      def self.method_injected_by_foo
        ...
      end
    end
  end
end

module Bar
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Foo # We need to include this dependency for Bar
  include Bar # Bar is the module that Host really needs
end

But why should Host care about Bar‘s dependencies, namely Foo? We could try to hide these from Host directly including Foo in Bar:

module Bar
  include Foo
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Bar
end

Unfortunately this won’t work, since when Foo is included, its base is the Bar module, not the Host class. With ActiveSupport::Concern, module dependencies are properly resolved:

require "active_support/concern"

module Foo
  extend ActiveSupport::Concern
  included do
    def self.method_injected_by_foo
      ...
    end
  end
end

module Bar
  extend ActiveSupport::Concern
  include Foo

  included do
    self.method_injected_by_foo
  end
end

class Host
  include Bar # It works, now Bar takes care of its dependencies
end

Prepending concerns

Just like include, concerns also support prepend with a corresponding prepended do callback. module ClassMethods or class_methods do are prepended as well.

prepend is also used for any dependencies.

Methods
C
I
P

Instance Public methods

class_methods(&class_methods_module_definition)

Define class methods from given block. You can define private class methods as well.

module Example
  extend ActiveSupport::Concern

  class_methods do
    def foo; puts 'foo'; end

    private
      def bar; puts 'bar'; end
  end
end

class Buzz
  include Example
end

Buzz.foo # => "foo"
Buzz.bar # => private method 'bar' called for Buzz:Class(NoMethodError)

Source: show | on GitHub 

# File activesupport/lib/active_support/concern.rb, line 209
def class_methods(&class_methods_module_definition)
  mod = const_defined?(:ClassMethods, false) ?
    const_get(:ClassMethods) :
    const_set(:ClassMethods, Module.new)

  mod.module_eval(&class_methods_module_definition)
end

included(base = nil, &block)

Evaluate given block in context of base class, so that you can write class macros here. When you define more than one included block, it raises an exception.

Source: show | on GitHub 

# File activesupport/lib/active_support/concern.rb, line 158
def included(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_included_block)
      if @_included_block.source_location != block.source_location
        raise MultipleIncludedBlocks
      end
    else
      @_included_block = block
    end
  else
    super
  end
end

prepended(base = nil, &block)

Evaluate given block in context of base class, so that you can write class macros here. When you define more than one prepended block, it raises an exception.

Source: show | on GitHub 

# File activesupport/lib/active_support/concern.rb, line 175
def prepended(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_prepended_block)
      if @_prepended_block.source_location != block.source_location
        raise MultiplePrependBlocks
      end
    else
      @_prepended_block = block
    end
  else
    super
  end
end