Fix polymorphic type setter

[Exelord]

Hey @lgebhardt!
We encounter a problem associated with the polymorphic relation. Here is an example in code:

Setup

Models:

class Conversation < ActiveRecord::Base
  has_many :topics
end

class Account::Template < ActiveRecord::Base
  has_many :topics
end

class Topic < ActiveRecord::Base
  belongs_to :document, polymorphic: true
end

Resources:

class Template < JSONAPI::Resource
  model_name 'Account::Template'
  has_many :topics
end

class Topic < JSONAPI::Resource
  has_one :document, polymorphic: true
end

Usage

Request: [ POST ] /topics

{
    "data": {
        "attributes": {
            "name": "Topic name",
        },
        "relationships": {
            "document": {
                "data": {
                    "type": "templates",
                    "id": "1"
                }
            },
        },
        "type": "topics"
    }
}

Problem

Newly created topic has the wrong name of document_type filed.
After typing topic.document_type it was Templates but in fact, the polymorphic type should be taken from a model name and it should look like Account::Template because it's the real name of the resource's model's class name.

Solution

I overwrite a method which assigns polymorphic type, and I created a method which looking for a real model class name of given relation's key_type.

I hope it will help :)

[barelyknown]

Nice catch. Can you add a test that fails without this fix and passes with it?

[Exelord]

@barelyknown I added a test. I'm not familiar with the rails test framework (I used to use rspec) so feedback is welcome.

[barelyknown]

Nice and simple - thanks!