I18n ActiveModel Subclass Lookup

Given the following Class definitions:

class Conversation
  class Message
    include ActiveModel::Validations
    attr_accessor :quantity
    validates :quantity, :presence => true
  end
end

There is no way to reference the error messages for the Message class. For example the correct lookup for the class Conversation would be:

activemodel:
  errors:
    models:
      conversation:
        attributes:
          quantity:
            blank: "Some custom message"

But what is it for the Message class? I tried:

activemodel:
  errors:
    models:
      conversation:
        message:
          attributes:
            quantity:
              blank: "Some custom message"

activemodel:
  errors:
    models:
      message:
        attributes:
          quantity:
            blank: "Some custom message"

activemodel:
  errors:
    models:
      conversation::message:
        attributes:
          quantity:
            blank: "Some custom message"

None of them work.

If this is a bug then I would vote for the following to be the correct lookup:

activemodel:
  errors:
    models:
      conversation:
        message:
          attributes:
            quantity:
              blank: "Some custom message"