problem

There is no easy way to join tables for filtering. This should be changed.

solution

1. Implement it so that 1-N tables can be filtered
2. Implement it so that N-N (or 1-N-1) can be filtered
   Implement that with a new Filter_Join class which accepts another foreign table: foreign table 1: the assoc table, foreign table 2: the other table pointing to the assoc table