Polymorphic relationships via mixin break tests

[Asherlc]

It looks like when you have a polymorphic Ember Data relationship that uses a mixin as the common ancestor, moduleForModel breaks, as it tries to resolve the mixin name to a model. needsing it doesn't appease it, for obvious reasons.

See the example here:
https://ember-twiddle.com/5c95f54a6aff631595c0323aca06eea6?openFiles=controllers.application.js%2C

[pangratz]

If the relationship would be a model, you would need to needs: ['model:other-model']. If it's a polymorphic relationship using mixins, you need to needs that mixin via, in your case, needs: ['mixin:child']. See this updated twiddle.