Context

Django serializes related model references (e.g. ForeignKey, ManyToManyField) as strings, [e.g. field=models.ForeignKey(on_delete=..., to='polls.question')] rather than as class references [as is currently the case for embedded models, e.g. field=django_mongodb_backend.fields.EmbeddedModelArrayField(embedded_model=mongo_examples.models.Tag)] so that migrations don't need to be adjusted if those classes are later removed.

Definition of done

All related model references (EmbeddedModelField,  EmbeddedModelArrayField, PolymorphicEmbeddedModelField, PolymorphicEmbeddedModelArrayField) are serialized as strings.