Inherited resources not rendering parent's attributes (w/ Null Adapter and polymorphism)

[natonnelier]

I have the following Resource:

module Api::V2
  class RecognitionResource < BaseResource
    self.adapter = Graphiti::Adapters::Null
    self.polymorphic = [BarcodeResource, TokenResource]

    belongs_to :page, foreign_key: :page_id, except: [:writable], resource: PageResource

    attribute(:confidence, :float)
    attribute(:coordinates, :hash)
    attribute(:kind, :string)
    attribute(:value, :string)
    attribute(:id, :string)
    attribute(:page_id, :string)
  end
end

From which this resource inherits:

module Api::V2
  class TokenResource < RecognitionResource
    self.model = Recognitions::Token
    self.type = "text"
end

I should add they don't use ActiveRecord models but instead plain ruby objects (with attr_accessors in place for every attribute).
Now, if I move the attributes and association to the child resource this works fine. But if I leave them in the parent resource it renders no attributes and the association is not loaded.

Is there some library I should import?
Thanks