I just happened to write: > >> Also it is possible to declare two fields in two different classes, e.g. > >> B1.a and B2.a, with the identical mapped-by pointing to the same field > >> of the same related class, e.g. A.a. That would mean that in theory two > >> FKs would have to be created for the same column, pointing to different > >> tables. That neither seems to make sense to me. > >> > > > > It makes sense to me. Same as above. > > > > It's not common to handle these associations in relational databases, but > > very common when using UML. > > > In UML you'd have to write something like "for any b1 of type B1 and b2 > of type B2: b1.a==b2.a" in OCL to express the above. Why should we > Sorry that was utter nonsense. After thinking about really, it rather seems to mean two mutually exclusive associations. So if an object of type A is referenced by a B1 object through B1.a, there can be no B2 object referencing it through B2.a (and vice versa).
Implementing this using mapped-by would firstly lead to problems due to impossibility of FK constraints, and secondly be one of the least obvious and understandable ways of implementing it ;)
