Subclass misses association defined on parent

[matthewd]

If an association is defined on the superclass after a subclass has been given an association of its own, the subclass will have the association methods present, but calling them will raise AssociationNotFoundError.

https://gist.github.com/matthewd/8b88b7356d4076fe6a10

[thedarkone]

To anyone that is going to work on this, I think this is very similar to #18926.

[akihiro17]

I guess the cause of this issue would be that
Association#associationmethod tries to get the reflection only in FancyPost._reflections.

Post._reflections['comments'] =>
#<ActiveRecord::Reflection::HasManyReflection:0x007fb24bb55b08 @name=:comments, @scope=nil, @options={}, @active_record=Post(id: integer), @klass=nil, @plural_name="comments", @automatic_inverse_of=nil, @type=nil, @foreign_type="comments_type", @constructable=true, @association_scope_cache={}, @scope_lock=#<Mutex:0x007fb24bb7df18>>

FancyPost._reflections['comments'] =>
nil

My workaround for this issue is shown below.

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations.rb#L153

def association(name) #:nodoc:
  association = association_instance_get(name)

  if association.nil?
    reflection = self.class._reflect_on_association(name)

    # if reflection is nil, try to get the parent class' reflection
    klass = self.class.superclass
    while reflection.nil? && klass.respond_to?(:_reflect_on_association)
      reflection = klass._reflect_on_association(name)
      klass = klass.superclass
    end

    unless reflection
      raise AssociationNotFoundError.new(self, name)
    end
    association = reflection.association_class.new(self, reflection)
    association_instance_set(name, association)
  end
  association
end

[rails-bot]

This issue has been automatically marked as stale because it has not been commented on for at least
three months.

The resources of the Rails team are limited, and so we are asking for your help.

If you can still reproduce this error on the 4-2-stable, 4-1-stable branches or on master,
please reply with all of the information you have about it in order to keep the issue open.

Thank you for all your contributions.

[dguettler]

Just ran into this issue when trying to delete a record which is a subclass. Above workaround mitigates the issue.
ActiveRecord: 4.2.5.1

# Good
class A < ActiveRecord::Base
end

class B < A
end

class A
  has_many :apples
end

B._reflections["apples"] => Reflection

# Bad
class A < ActiveRecord::Base
end

class B < A
  has_many :oranges
end

class A
  has_many :apples
end

B._reflections["apples"] => nil

[maclover7]

Opened #24719.

[joenio]

related to #6944

[brauliobo]

related to #21908