Add support for creating resources with polymorphic has one.

[barelyknown]

This simply adds to PR https://github.com/cerebris/jsonapi-resources/pull/255 from @ggordon. I added support for creating resources with a polymorphic has_one. I don't like the code, but I wanted to change as little as possible for now since I need this feature ASAP. We should refactor to handle the creation of all has_one relationships the same way (whether or not they're polymorphic), but I think that we can merge prior to that since we'll have good test coverage and the public API feels right.

[dgeb]

I'd prefer to see the HasOne check first, then check association.polymorphic? within that block. We could then follow suit for HasMany relationships later.

[barelyknown]

Ok, I added that.

[dgeb]

I'd eventually like to allow restrictions on possible types in a polymorphic relationship. This doesn't have to make it into this PR, but we should think about the interface. I suppose it could be a separate param from polymorphic: true, such as allowed_types.

[dgeb]

All in all, this looks like a promising start on polymorphism - nice work @ggordon and @barelyknown!

[barelyknown]

I'm going to take a crack at support for has_many polymorphism now. We'll see how it goes.

[barelyknown]

That last commit adds polymorphism to the "read" side of has_many relationships. I'm not sure why there are merge conflicts - I'm not seeing that on my side.

[ggordon]

@barelyknown I had merge conflicts in #255 with the Readme, from PR #277, those are probably the same.

[barelyknown]

Ugh. The conflicts are a mess now. I'll see if I can work it out.

[lgebhardt]

Sorry, I know. I'm going through them now. So if you want to hold off I'll see if I can get it all resolved.

[lgebhardt]

I pushed the new rebase/274 branch up.

I hope I got it all correct. All the tests pass, and there are the same two lines that are not covered.

[barelyknown]

Oh, good. Thanks for doing that - I know that it was a pain!

[lgebhardt]

@barelyknown It was my fault for not merging this earlier.

[lgebhardt]

Thanks!

Merged by #288

[arcreative]

@lgebhardt I'm having an issue on 0.8.3 where trying to include the polymorphic relationship results in JSONAPI: Could not find resource 'subject'. (Class Api::V1::SubjectResource not found). The resource is supposed to be variable, not understanding why it's looking for SubjectResource when polymorphic is set to true... If I force the type with has_one :subject, polymorphic: true, class_name: 'Feedback', the included record has a blank type (data[0].relationships.subject = {type: "", id: "76"}). Either I'm doing something wrong, or...

Resource definition:
has_one :subject, polymorphic: true
Fails with JSONAPI: Could not find resource 'subject'. (Class Api::V1::SubjectResource not found
has_one :subject, polymorphic: true, class_name: 'Feedback' # Obviously not ideal, just testing
Returns invalid payload when including resource, e.g. data[0].relationships.subject = {type: "", id: "76"}

Model definition:
belongs_to :subject, polymorphic: true
All polymorphic stuff seems to otherwise be working normally