Replicate:

Create two models with parent and children relationship.
Try to POST a new `parent with no children.

Expect: 201
Get: 400 The children field is required.

This can be solved by extending the serializer as described in this thread:
http://discuss.emberjs.com/t/ember-data-1-13-7-using-jsonapi-how-to-post-hasmany-relationships-in-one-payload/8727/7

However, I would expect the adapter to handle this somehow. But maybe it's entirely Ember's fault!