peewee.py 文件源码

def join(self, dest, join_type=None, on=None):
        src = self._query_ctx
        if on is None:
            require_join_condition = join_type != JOIN.CROSS and (
                isinstance(dest, SelectQuery) or
                (isclass(dest) and not src._meta.rel_exists(dest)))
            if require_join_condition:
                raise ValueError('A join condition must be specified.')
        elif join_type == JOIN.CROSS:
            raise ValueError('A CROSS join cannot have a constraint.')
        elif isinstance(on, basestring):
            on = src._meta.fields[on]
        self._joins.setdefault(src, [])
        self._joins[src].append(Join(src, dest, join_type, on))
        if not isinstance(dest, SelectQuery):
            self._query_ctx = dest