Issue with Updating the foreign key of a polymorphic relationship

Hello,

we encountered this issue with an setup like:

class Photo < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
  ...
end

class Order < ActiveRecord::Base
  ...
  has_one :photo, as: :imageable
  ...
end

class Api::V1::PhotoResource < JSONAPI::Resource
  ...
  has_one :imageable, polymorphic: true
  ...
end

class Api::V1::OrderResource < JSONAPI::Resource
  ...
  has_one :photo, polymorphic: true
  ...
end

The idea is, that you can upload a photo separately to the photo endpoint and then link it to the order while creating/updating it via relationships.

Some debugging revealed that it's recognised as polymorphic but replace_polymorphic_to_one_link is always called on the OrderResource, even with has_one :photo, polymorphic: true, foreign_key_on: :related in the OrderResource.

I would like to fix this behaviour myself, when somebody can point me in the right direction. Until then a workaround for this is:

def replace_polymorphic_to_one_link(relationship_type, relationship_key_value, relationship_key_type)
  @model.photo = Photo.find(relationship_key_value)
end

in the OrderResource.

Start with replace_polymorphic_to_one_link and the Photo, Order, Api::V1::PhotoResource, and Api::V1::OrderResource relationship definitions shown in the issue. Reproduce creating or updating an Order through relationships after uploading a Photo, then verify that the polymorphic link is handled on the correct resource without the OrderResource workaround.