Should Associations::Preloader allow subclasses not to define the association?

rubyonrails-core
1

Hi,

I am working a project where subclasses (using STI) of an model have different relationships. Quick Example:

class Course < ActiveRecord::Base

has_many :learning_units, as: :container

end

class LearningUnit <ActiveRecord::Base

end

class LessonGroup < LearningUnit

belongs_to :container, polymorphic: true

has_many :lessons,

        -> { where(container_type: LessonGroup.name) },

        class_name: "Lesson",

        foreign_key: "container_id"

end

class Lesson < LearningUnit

belongs_to :container, polymorphic: true

notice no lessons relationship

end

Everything works as expected except that when I try to do

course_instance.learning_units.includes(:lessons)

an exception is raised because one of the subclasses does not have a lessons relationship.

This is where it happens: https://github.com/rails/rails/blob/4-1-stable/activerecord/lib/active_record/associations/preloader.rb#L147

Unless I am mistaken, there’s no configuration to change this behaviour. A possible fix would be to raise the exception only if none of the classes define the requested associations. Is there any chance such a change gets merged?

Any thoughts are more than welcome.