Cross join prevention in base query

[nZac]

Investigate if it is possible to override the base query object such that cross joins would raise an exception unless done explicitly. If this works, we should try to factor it out and put it in keg elements

NOTE: This is xpost from an internal library. The initial work was done by @bladams

[rsyring]

Just FYI, Keg doesn't provide the base query. That comes from Flask-SQLAlchemy. We could certainly enhance/replace it.

I'd say the first step to resolving this is to ask on the SQLAlchemy mailing list if it's possible. But maybe even before that we should define a good example use case.