Model inheritance or include

inherit_include.rb

# Structure for inheritance
# app/models/parent/base.rb

class Parent::Base
  include Mongoid::Document
end

# app/models/parent/first_child.rb
class FirstChild < Parent::Base
  # fields needed for first child
end

app/models/parent/second_child.rb
class SecondChild < Parent::Base
  # fields needed for second child
end

# Structure for Include/Extend
# 
# app/models/parent.rb

class Parent 
  include Mongoid::Document

  include Parent::FirstInclude
  include Parent::SecondInclude
  # fields

end

# app/models/parent/first_include.rb

module Parent::FirstChild
  extends ActiveSupport::Concern

  included do
  end

  module ClassMethods
  end
  module InstanceMethods
  end
end
# similarly app/models/parent/second_include.rb