WIP - try to get to minimize the code divergence between hasMany with or
without polymorphism

The serializer will return consistently a id-type tuple

TODO:
* 40 failures
WIP - I don't know why for the moment, but embedded hasMany returns a
reference instead of a id-type tuple

TODO:
* 29 failures
* Why do I get a reference in hasRelationship with an embedded
* association