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::Concernthe above module could instead be written as:

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 Foomodule and a Barmodule 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 Hostcare about Bar's dependencies, namely Foo? We could try to hide these from Hostdirectly including Fooin 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 Foois included, its baseis the Barmodule, not the Hostclass. With ActiveSupport::Concern, module dependencies are properly resolved:

require 'active_support/concern'

module Foo
  extend ActiveSupport::Concern
  included do
    class_eval do
      def self.method_injected_by_foo
        ...
      end
    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
Methods
A
E
I
Class Public methods
extended(base)
# File activesupport/lib/active_support/concern.rb, line 103
def self.extended(base)
  base.instance_variable_set("@_dependencies", [])
end
Instance Public methods
append_features(base)
# File activesupport/lib/active_support/concern.rb, line 107
def append_features(base)
  if base.instance_variable_defined?("@_dependencies")
    base.instance_variable_get("@_dependencies") << self
    return false
  else
    return false if base < self
    @_dependencies.each { |dep| base.send(:include, dep) }
    super
    base.extend const_get("ClassMethods") if const_defined?("ClassMethods")
    if const_defined?("InstanceMethods")
      base.send :include, const_get("InstanceMethods")
      ActiveSupport::Deprecation.warn "The InstanceMethods module inside ActiveSupport::Concern will be "              "no longer included automatically. Please define instance methods directly in #{self} instead.", caller
    end
    base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
  end
end
included(base = nil, &block)
# File activesupport/lib/active_support/concern.rb, line 125
def included(base = nil, &block)
  if base.nil?
    @_included_block = block
  else
    super
  end
end