Why use ActiveSupport::Concern instead of just a plain module? - ruby-on-rails

Question

I never understood why one has to use ActiveSupport::Concern is used for mixins instead of a plain module. Is there a simple answer to what ActiveSupport::Concern provides (at least in Rails 5) that a simple module without using ActiveSupport::Concern will do?

Answer 1

From https://api.rubyonrails.org/classes/ActiveSupport/Concern.html:
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