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.

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) 

# 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

# 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

# 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