Implement parent select-expression with union parent
Implement parent select-expression with union parent

currently only union-parent fields could be returned. now it's possible
to add select-expressions (for filtering) based on union-source-model
data/relations.
                                Db
                              +---------+
  Db             Union    +-->+ PnUsers |...
+----------+   +-------+  |   +---------+
| Attempts +-->+ Users +--+     Db
+----------+   +-------+  |   +----------+
                          +-->+ KwfUsers |...
                              +----------+

select Attempts where Parent('Users', ...)

TODO: need to test if it also works for union-model-relations

koala-framework/koala-framework,koala-framework/koala-framework