а зачем вам сущность
вроде в запросе можно указывать что должно быть возвращено сразу
Using this feature maximises HTTP cache hits for your API resources (HTTP/2 allows a server to pre-emptively send (or "push") responses (along with corresponding "promised" requests) to a client in association with a previous client-initiated request. )
By default, the serializer provided with API Platform represents relations between objects using dereferenceable IRIs. They allow you to retrieve details for related objects by issuing extra HTTP requests.
Note: We strongly recommend using Vulcain instead of this feature. Vulcain allows creating faster (better hit rate) and better designed APIs than relying on compound documents, and is supported out of the box in the API Platform distribution.