Scoping a `has_one` relationship

After searching for a while, the answer here isn't obvious to me, so I thought I'd bring it up as an issue for reference.

I have 3 models:

class User < ActiveRecord::Base
  has_many :answers
end

class Question < ActiveRecord::Base
  has_many :answers
end

class Answer < ActiveRecord::Base
  belongs_to :user
  belongs_to :question
end

What I'd like to do is include the current_user_answer in a questions#show request, like so: /api/questions/123?include=current_user_answer. It seems like I should be able to write my QuestionResource as follows:

class QuestionResource < JSONAPI::Resource
  has_many :answers

  has_one :current_user_answer, class_name: 'Answer'

  # doesn't get called
  def self.record_for_current_user_answer(options={})
        current_user = options[:context][:current_user]
        AnswerResource.records(options).find_by(
          question: @model,
          user: current_user
        )
  end

  # doesn't get called either
  def current_user_answer(options={})
        current_user = options[:context][:current_user]
        AnswerResource.records(options).find_by(
          question: @model,
          user: current_user
        )
  end
end

Neither of these methods get called; instead, I get an exception "Association named 'current_user_answer' was not found on Question; perhaps you misspelled it?" Well, by definition here I'm trying to avoid writing a query method directly on the model, and anyways such a query method or association would need to get the context passed in from the enclosing Resource.

In short, there seems to be no documented way to scope an included associated to something in the context. Or am I missing something? What's the canonical way to get this related resource without an extra request?

Start with the QuestionResource relationship declaration and the /api/questions/123?include=current_user_answer request described here; reproduce the association-not-found exception and trace how included has_one relationships are resolved. Determine whether context-scoped lookup is supported, and document or implement a canonical way to return the related Answer without an extra request.