collections = relation('Collection', secondary=account_to_collection_map,
    backref='accounts')

def get_other_collections(self, maxrows=None):
    """ Returns the collections this Account does not have yet.  """
q = Session.object_session(self).query(Collection)
q = q.filter(~Collection.accounts.any(id=self.id))
sqlalchemy многие ко многим, но наоборот?