关于 sqlalchemy 中 desc 的疑问

[honmaple]

关于 desc 我见过两种用法,分别是 order_by(Questions.time.desc())和 order_by(desc(Questions.time))，请教这两种用法有什么区别？
现在我要设置关联表的排序，应该使用哪种方法:

tag_question = db.relationship('Tags',
                               secondary=tag_question,
                               backref=db.backref('questions',
                                                  lazy='dynamic',
                                                  order_by='desc(Questions.time)')
                               )

还是

tag_question = db.relationship('Tags',
                               secondary=tag_question,
                               backref=db.backref('questions',
                                                  lazy='dynamic',
                                                  order_by='Questions.time.desc()')
                               )

但实际上 desc 并没有起作用(是我的错觉吗)

如果我要设置默认排序

__mapper_args__ = {
        "order_by":'Questions.time desc'
    }

却只有这种方法勉强有效,原来的两种方法直接运行不了,而Questions.time desc虽然可以运行，但是有错误提示

/venv/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py:575: SAWarning: Can't resolve label reference 'Questions.time desc'; converting to text() (this warning may be suppressed after 10 occurrences)
util.ellipses_string(element.element))

求助大神

[honmaple]

设置默认排序使用
__mapper_args__ = {
"order_by":time.desc()
}