Concerns in Rails 3

A pattern you see around in Rails 3 source code is

module Foo
  extend ActiveSupport::Concern
  # ...
end

Concern

1. solves a problem of module dependencies piling up on the host, and
2. provides a pattern for extending the host.

Piling up dependencies becomes a problem when you have a module A and module B, and B uses methods provided by A at include time. Usually, host class C needs to include module A before module B so that needed methods are available at include time, i.e. piling up dependencies.

With ActiveSupport::Concern you can include A to B and just B to host C, cascading dependencies.

module A
  extend ActiveSupport::Concern
  # ...
end

module B
  extend ActiveSupport::Concern
  include A
  
  included do
    # use methods provided by A
  end
end

class C
  include B
end

A concern module has a modified Module#included which can be passed a block to be evaluated. It also automatically extends its ClassMethods module and includes its InstanceMethods module to host, if present.

See also documentation in the source code.