Clarify API usage for relationship properties (potential change) #8607
Description
At the moment when trying to create/edit entities via the API, relationship fields based on the same relationship type/property are forced to have the same value. This is good in terms of server side validation for UI, but not so convenient when just using the API.
Ideally just one of the fields receives the value and the rest copy the value. In fact, for multiple-inheritance, we could have a single denormalized relationship structure.