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?

Start in lib/jsonapi/resource.rb at the generated relationship method shown in the issue, comparing the polymorphic and non-polymorphic branches. Trace how get_related_resource reaches this method and verify that the model instance can select the intended resource subclass without breaking existing relationship behavior.