Question about ActiveSupport::Concern and mongoid model definition

I am using mongoid and MongoDB and have come lately to a very tough
problem and I need an advice.

I am working on a CMS and one of the ideas was that CMS would provide
some basic models definitions and end user would, if needed, extend
basic class with its own extension and save them to different collection

class DcPage
include Mongoid::Document

field a …
belongs_to b …
validates a …

class MyPage < DcPage
field c …
validates c …

Up until last version of mongoid this worked (with little hack) and data
would be saved to my_pages collection. Because of some problem, they no
longer support this behaviour and data always gets saved to dc_pages

When explaining my problem, they suggested that I use
ActiveSupport::Concern and provided me with an example. Which works
perfectly OK if extended class is defined in same source file. Which
btw. never happens in praxis.

module CommonBehaviour
extend ActiveSupport::Concern

included do
field :subject, type: String, default: ‘’
# …

class DcPage
include Mongoid::Document
include CommonBehaviour

class MyPage
include Mongoid::Document
include CommonBehaviour

So far I have found out that it works if I require basic source file in
my second file. Which looks like this:
require '/some/path/to/my/gem/app/models/dc_page.rb

You can see my pain. Basic source file would of course be backed into
gem and therefor becomes a moving target.

Please help me with better solution.