Flask Restless 属性错误：'NoneType' 对象没有 '_sa_instance_state' 属性

Question

我现在正在使用 Flask、Flask-SQLAlchemy 和 Flask-Restless 来为一个小型的私有 AngularJS 应用提供 REST 接口。到目前为止，我已经能够有效地创建和使用一些不涉及关系的接口。

不过，我有几个表格需要和其他表格建立关系，这就是我遇到问题的地方。当我向 Flask-Restless 接口发送 POST 请求时，出现了以下 Flask 错误：

  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python2.7/dist-packages/flask_restless/views.py", line 145, in decorator
    return func(*args, **kw)
  File "/usr/local/lib/python2.7/dist-packages/mimerender.py", line 227, in wrapper
    result = target(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/flask/views.py", line 84, in view
    return self.dispatch_request(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/flask/views.py", line 149, in dispatch_request
    return meth(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/flask_restless/views.py", line 1263, in post
    self.session.add(instance)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/scoping.py", line 150, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 1492, in add
    self._save_or_update_state(state)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 1510, in _save_or_update_state
    halt_on=self._contains_state):
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/mapper.py", line 2457, in cascade_iterator
    visited_states, halt_on))
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/relationships.py", line 1449, in cascade_iterator
    get_all_pending(state, dict_)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/dynamic.py", line 185, in get_all_pending
    c.all_items
AttributeError: 'NoneType' object has no attribute '_sa_instance_state'

我觉得这个错误可能和我的模型有关。以下是我当前的模型定义：

surveyOptions = db.Table('survey_options',
    db.Column('survey_id', db.Integer, db.ForeignKey('survey.id')),
    db.Column('option_id', db.Integer, db.ForeignKey('option.id')),
    db.Column('count', db.Integer)                      
    )

class Survey(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    category = db.Column(db.String(50))
    question = db.Column(db.Text)
    startDate = db.Column(db.DateTime)
    endDate = db.Column(db.DateTime)
    options = db.relationship('Option', secondary=surveyOptions,
        backref=db.backref('surveys', lazy='dynamic'), lazy='dynamic')

def __init__(self, category, question, startDate=None, endDate=None, options=None):
    self.category = category
    self.question = question
    if startDate == None:
        self.startDate = datetime.utcnow()
    if endDate == None:
        self.endDate = datetime.utcnow()
    self.options.append(options)

class Option(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    text = db.Column(db.Text)

    def __init__(self, text):
        self.text = text

...

manager = flask.ext.restless.APIManager(app, flask_sqlalchemy_db=db)
manager.create_api(Survey, methods=['GET','POST','PUT','DELETE'], include_columns=['id','category', 'question', 'startDate','endDate', '_options.id','options.text']);

我非常希望能有人帮我找出这个错误的原因。我不太确定接下来该如何排查这个错误。

如果需要，我可以提供更多前端和后端的代码示例。

Answer 1

这是因为在这一部分，你似乎是在用一个列表去添加另一个列表：

def __init__(self, category, question, startDate=None, endDate=None, options=None):
    self.category = category
    self.question = question
    if startDate == None:
        self.startDate = datetime.utcnow()
    if endDate == None:
        self.endDate = datetime.utcnow()
    self.options.append(options)  # assuming options is a list, this will fail.

这个问题的原因很简单。如果我有一个选项列表：

foo = ['one', 'two']

然后我想再添加一些选项：

bar = ['three', 'four']

如果我用 foo.append(bar)，我得到的是：

foo = ['one', 'two', ['three', 'four']]

而你其实想要的是 foo + bar，这样会得到：

foo = ['one', 'two', 'three', 'four']

当你解决了这个问题后，你还会遇到第二个更奇怪的问题——如果在创建 Survey 对象时不指定 options，那么你的代码会尝试对对象执行 self.options.append(None)。你需要对此进行检查：

if choices:  # None or [] will evaluate to False
    self.choices += choices

我还注意到你在用 == 操作符来比较 None，其实应该用 is 来做这些比较，比如 if startDate is None:。