use peewee can't sort by fields

[axiaoxin]

Traceback (most recent call last):
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/gunicorn/workers/base_async.py", line 56, in handle
    self.handle_request(listener_name, req, client, addr)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/gunicorn/workers/ggevent.py", line 160, in handle_request
    addr)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/gunicorn/workers/base_async.py", line 107, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/werkzeug/contrib/fixers.py", line 152, in __call__
    return self.app(environ, start_response)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 2295, in wsgi_app
    response = self.handle_exception(e)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 1741, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask_admin/base.py", line 69, in inner
    return self._run_view(f, *args, **kwargs)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask_admin/base.py", line 368, in _run_view
    return fn(self, *args, **kwargs)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask_admin/model/base.py", line 1882, in index_view
    view_args.search, view_args.filters, page_size=page_size)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask_admin/contrib/peewee/view.py", line 399, in get_list
    query, joins = self._order_by(query, joins, sort_field, sort_desc)
  File "/data/playground/projects/zhiyun_notifer/deploy/venv/lib/python2.7/site-packages/flask_admin/contrib/peewee/view.py", line 324, in _order_by
    if sort_field.model_class != self.model:
AttributeError: 'DateTimeField' object has no attribute 'model_class'