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Type:
Module

A typical module looks like this:

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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:

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require 'active_support/concern'
 
module M
  extend ActiveSupport::Concern
 
  included do
    scope :disabled, -> { where(disabled: true) }
  end
 
  module ClassMethods
    ...
  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:

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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:

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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:

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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 # works, Bar takes care now of its dependencies
end
included
  • References/Ruby on Rails/Rails/Classes/ActiveSupport/ActiveSupport::Concern

included(base = nil, &block) Instance Public methods

2025-01-10 15:47:30
append_features
  • References/Ruby on Rails/Rails/Classes/ActiveSupport/ActiveSupport::Concern

append_features(base) Instance Public methods

2025-01-10 15:47:30