[web2py] smartgrid and unsupported query error

[weheh]

I'm running SQLFORM.smartgrid with a left= join as follows:

    grid = SQLFORM.smartgrid(
               db.url_queue,
               constraints=dict(url_queue=query),
               left=db.brand.on(
                   (auth.accessible_query('read', db.brand, auth.user_id)) &
                   (db.brand.id == db.url_queue.brand_id)
                   ),
               fields=[...],
               headers={...},
               paginate=20,
               maxtextlength=100,
               searchable=True,
               deletable=True,
               editable=False,
               details=False,
               create=False,
               ui='web2py',
               linked_tables=[],
               )
The left join is causing smartgrid to give me an "unsupported query" error 
message. If I comment out the left= arg, the error message goes away but 
the query returns the wrong data, as expected. Any help would be appreciated
. Thanks.