WFS output does not leverage JOIN objects

[sdlime]

Posted originally to mapserver-users by Tom. Problem is that WFS query output
does not leverage the JOIN objects for a layer. 

Outputting them is relatively simple. The bigger problem is that anything you
output needs to be queryable via a GetFeature request. At present there is
nothing in the MapServer query code to leverage joined attributed. Handling
one-to-many joins is even more problematic. Not impossible, but not trivial
either... 

I really wish there was some notion of "read-only" attributes- that is
attributes you can get at but not query upon. That would allow us to use GML
templates as well. *sigh*

Steve