Rails: Including a Concern with a constant within a Concern

Question

I have concern in which I store constants:

module Group::Constants   extend ActiveSupport::Concern    MEMBERSHIP_STATUSES = %w(accepted invited requested     rejected_by_group rejected_group) end 

And another concern that I wish to use these constants:

module User::Groupable   extend ActiveSupport::Concern   include Group::Constants    MEMBERSHIP_STATUSES.each do |status_name|     define_method "#{status_name}_groups" do       groups.where(:user_memberships => {:status => status_name})     end   end end 

Unfortunately, this results in a routing error:

uninitialized constant User::Groupable::MEMBERSHIP_STATUSES 

It looks like the first concern isn't loading properly in the second concern. If that's the case, what can I do about it?

Answer 1

It appears this behavior is by design, as explained nicely over here.

What you'll need to do in this case is not have Group::Constants extend from ActiveSupport::Concern since that will block its implementation from being shared with other ActiveSupport::Concern extending modules (although it will be ultimately shared in a class that includes the second module):

module A   TEST_A = 'foo' end  module B   extend ActiveSupport::Concern   TEST_B = 'bar' end  module C   extend ActiveSupport::Concern   include A   include B end  C::TEST_A  => 'foo' C::TEST_B  => uninitialized constant C::TEST_B  class D   include C end  D::TEST_A  => 'foo' D::TEST_B  => 'bar' 

In short, you'll need to make Group::Constants a standard module and then all will be well.

Answer 2

If you want to keep everything in one file, and if you can stomach a bit of boilerplate, you could break up your module into a "concern" bit and a "non-concern" bit:

module A   FOO = [22]    def self.included base     base.include Concern   end    module Concern     extend ActiveSupport::Concern      class_methods do       def foo_from_the_perspective_of_a_class_method_in_A         {lexical: FOO, instance: self::FOO}       end     end   end end  module B   extend ActiveSupport::Concern    include A    FOO += [33]    def foo_from_the_perspective_of_an_instance_method_in_B     FOO   end end  class C   include B end   C.foo_from_the_perspective_of_a_class_method_in_A => {:lexical=>[22], :instance=>[22, 33]} C.new.foo_from_the_perspective_of_an_instance_method_in_B => [22, 33] C::FOO => [22, 33]