Control Embedded Behavior

[ambsw-technology]

By default, the HalModelSerializer classifies all nested BaseSerializer instances as embedded nested objects. In reality, the REST representation need not align 1:1 with the database representation so it should be possible to override this behavior.

For example, I'm supporting a 3rd party data specification (HL7v3) which has a 1:many relationship between Patient and ID (i.e. each patient can have multiple IDs). For indexing reasons, this means that IDs need to be in their own table, but they're not really intended as embedded objects (e.g. they aren't independently linkable).

This PR creates a HalPromoteEmbeddedMixin that can be added to a nested serializer to indicate that it should be stored at the top level of the representation (and not _embedded).

P.S. I really wanted to add a check to __init__ to make sure the Meta.list_serializer_class also uses HalPromoteEmbeddedMixin. If you don't add it in both places, the many=True serializer won't pick up the behavior. The problem is (1) I don't want to raise an exception because you don't have to mess with it if you never use many and (2) we don't import logging anywhere in the package so I didn't have a standard pattern to follow.

[houndci-bot]

Black would make changes.

[ambsw-technology]

not sure what it's complaining about but here's another thread on the odd message houndci/hound#1769