🚧 [#105] added broken polymorphic-serializer actor with tests

[bart-maykin]

fixes #105

Changes:

Changed the actor HyperlinkedModelSerializer to PolymorphicSerializer and updated tests acoridingly
Removed Medewerker, GeautomatiserdeActor en Org. Eenheid resources

[annashamray]

I have some comments how to make it work:

1. PolymorphicSerializerMetaclass dynamically createactor_identificaties a serializer for group field (here it's actor_identificatie). It uses related fields of Actor model to construct source attribute. For example, if actor_type = 'medewerker', then it will create a serializer class with a field actor_identificatie with source = medewerker.

   If you have OneToOne relations between Actor and Medewerker models, this would work. But you have FK relations, i.e. actor.medewerker is a manager and not a model.instance. To fix it I suggest any of two options:

   • change PolymorphicSerializerMetaclass implementation (this line https://github.com/maykinmedia/commonground-api-common/blob/ce9f32fb82089ad1bb73427e560d960712aab523/vng_api_common/polymorphism.py#L143)
   • use OneToOne relations. It will also fix a lot of django warnings, which appear when you start runserver, because django doesn't like ForeignKey(unique=True) thing

2. I don't think PolymorphicSerializerMetaclass supports cases then discriminator model and serializer field have different names. So I'd just rename soort_actor to actor_type, since it's the easiest

[annashamray]

3. PolymorphicSerializer doesn't have create andupdate, so you need to implement them by yourself if you want this resource to be writeable. For read-only resources you can leave it as it is.
4. NestedGegevensGroepMixin and PolymorphicSerializer don't work together. I'd suggest removing NestedGegevensGroepMixin, because you need to write custom create andupdate anyway