This constraint was thought-out to be used directly on a entity. But in several cases, i used it without a doctrine entity, for example when the form has fields for diferents entities.

In this case i want to have the way to specify not only the field also the entity that belongs the field.

I think that the change will be easy, so i will send a PR soon.

I think that the PR will be on 2.8 it is?

Regards!