MODEL_FACTORY_INJECTIONS + no double extend + polymorphic may have issues

[stefanpenner]

With double extend off in new ember Ember.ENV.MODEL_FACTORY_INJECTIONS isn't accurate, and most likely is causing issues.

Most likely:

data/addon/-debug/index.js

Lines 37 to 39 in e6cb564

	if (Ember.MODEL_FACTORY_INJECTIONS) {
	modelClass = modelClass.superclass;
	}

And likely others:
https://github.com/emberjs/data/search?utf8=%E2%9C%93&q=MODEL_FACTORY_INJECTIONS&type=

We should review and audit each occurrence, and address issues that arise.

Plan

• add tests for assertPolymorphicType

  data/addon/-debug/index.js

  Lines 37 to 39 in e6cb564

  	if (Ember.MODEL_FACTORY_INJECTIONS) {
  	modelClass = modelClass.superclass;
  	}

• add tests with both double extend and no double extend
• fix funky things discovered

[stefanpenner]

I believe ember (when double extend is disabled) needs to force MODEL_FACTORY_INJECTIONS to always be false: emberjs/ember.js#15204

[runspired]

There is an open PR for this I should fix at some point soon: (#4886)

[runspired]

We were never able to fix this because there isn't a good way to deal with creating an extra extend just-in-time. That said the need for doing so has greatly diminished with time, and the lack of reports indicates that we've mostly eliminated the footguns. Internally I don't believe we use Model.modelName anywhere at this point. Even adapters and serializers are moving away from it as we now provide them a schema wrapper.

For polymorphism, we still do have a check that walks the prototype chain, but it uses instanceof now. (and we'll be deleting the check soon because we've shown it to be unnecessary).

Going to close this as I don't think there's any further action for us to take directly on this issue.