I'm trying a way to add nodes to a group that doesn't require an explcit
check to see if the node is already there.  There are several parts to
this:
1) A UNIQUE constraint needs to be added to the association table as we
have in Django.  Something like:
CREATE UNIQUE INDEX db_dbgroup_dbnodes_dbgroup_id_dbnode_id_key ON public.db_dbgroup_dbnodes (dbgroup_id, dbnode_id);
2) Add the nodes one by one and flush after each one which will cause
the IntegrityError to appear if the constraint is violated
3) Wrap the append/flush in a nested session so that it is automatically
rolled back in the case of a constraint violation
4) finally after everything do the final commit (because flush just puts
it in SQLs buffers)

NOTE: Before this gets considered for a merge we have to find a way to
put the constraint in the model.  I didn't see how to do this.