Could _add_relationship generated methods use resource_for_model instead of resource_for?

In an application I'm working on, we had an issue where we had a model with a polymorphic belongs to association, that used different resource subclasses based on the type of the association.

For the basic index, show, etc. actions this worked fine (you can override resource_type_for or use model hints), but when we tried to access one of these models using a get_related_resource route, we found that it used the superclass resource.

This seems to be because of this method in resource.rb:

define_method relationship_name do |options = {}|
  relationship = self.class._relationships[relationship_name]

  if relationship.polymorphic?
    associated_model = public_send(associated_records_method_name)
    resource_klass = self.class.resource_for_model(associated_model) if associated_model
    return resource_klass.new(associated_model, @context) if resource_klass
  else
    resource_klass = relationship.resource_klass
    if resource_klass
      associated_model = public_send(associated_records_method_name)
      return associated_model ? resource_klass.new(associated_model, @context) : nil
    end
  end
end unless method_defined?(relationship_name)

Using relationship.resource_klass looks up the resource class using the class of the model, which doesn't allow for the resource class to be changed based on attributes of the model. At this point, the model instance is known, so it should be possible to adapt it to look up the resource class based on the model instance instead of the model class.

Currently, we're getting around this by overriding .new on the superclass relationship like this, but it's pretty hacky:

def self.new(model, *args)
  return super if self < V1::QuestionResource || model.content_type.nil?
  resource_for_model(model).new(model, *args)
end

Is this something there'd be interest in changing?

lib/jsonapi/resource.rb にある、issue に示された生成されたリレーションメソッドから始め、ポリモーフィック分岐と非ポリモーフィック分岐を比較します。get_related_resource がこのメソッドに到達する経路を追跡し、既存のリレーション動作を壊すことなく、モデルインスタンスが意図したリソースサブクラスを選択できることを確認します。