From Java Persistence with Hibernate:
"You probably won't need to use the theta-style joins often. Note
that it's currently not possible in HQL or JPA QL to outer join two
tables that don't have a mapped association -- theta-style joins are
When dealing with larger domain models (say, anything more than 100
entities) the number of permutations concerning how you could
potentially want to correlate data in the system grows rapidly.
However, the theta-style restriction makes providing a flexible
mechanism for reporting difficult.
I want to be judicious in my use of explicit relationship mappings, but
this restriction seems to imply the exact opposite - that I'd have to
use them freely in more places than I might have originally wanted to,
and create an explicit mapping for everything that any reporting query
might be correlated on.
Would it be possible to relax this restriction, and allow the
theta-style query to join on things that aren't explicitly mapped?