Add validation for "Primary key already exists"

[javier-godoy]

When the entity id is not automatically generated, it's possible that an insert will trigger a primary key violation. That's a pretty common case, and a custom validator would be needed in order to avoid reporting it as an "unknown error".

Should we provide some support for this scenario in commons-backend? (either as an ad-hoc validator, or as some exception postprocessing.)

[ngonzalezpazFC]

Detecting this while trying to save an existing ID could be useful to force the user to use the right method (save or update) for the action that he's trying to do, because if you use save with an existing ID, the entity will be updated. Maybe the user tried to persist a new entity but ended up updating an existing one, without noticing it.

[mlopezFC]

I'm not sure if Commons Backend should do something in this scenario. Whether the entity id is automatically generated or not is a JPA mapping decision. Implementing this would require to "configure" the library in a "automatically generated id" mode so the validation is useful or not.