Q() objects cannot be mixed between models
This has been fixed in 3f6336bba6d3a0bda691fe3e333767008c2308bb

It is not possible to create an "OR" query comparing columns from two tables.  This is due to the way where clauses are created programmatically:

```
# where clauses between models are separated by calls to join() or switch()
Blog.select().where(Q( ??? )).join(Entry).where( Q(???) )
```

With the new .filter() syntax, it is syntactically possible to create these types of queries:

```
Blog.filter(Q(title='my blog') | Q(entry__title='some special entry'))
```

But there is no underlying mechanism to generate the right SQL since where clauses are stored in a dictionary keyed by the model they pertain to.

Fixing this will most likely require significant rewriting.
