Various details on how to work with persistence.
Entity extensions cannot be directly mapped. The main entity should be used instead of its extension in mapping. For example:
We have an entity of class A:
And we have its extension of class B:
In our particular use case we might want to map some other entity field to B:
The correct use case looks like:
This behavior is due to the fact that entity extension is not a separate entity. It is just brings some additional fields to its main entity. That is why it cannot be used in relationship mapping as separate entity.