Flask-Restful Endpoint投掷werkzeug.routing.BuildError

Question

我正在使用Flask-Restful和SQLAlchemy来开发一个API，并且遇到了一个我有解决方法的行为，但我不确定坚持使用这个解决方案是一个很好的长期策略。

相当标准的API，带有POST功能的List项目资源和带有GET功能的项目单一资源。我通过curl和web表单测试了POST函数，并收到以下错误消息：

BuildError: Could not build url for endpoint 'brand' with values ['_sa_instance_state']. Did you forget to specify values ['brand_symbol']?

奇怪的是，添加一个print语句（请参阅下面的Resource声明末尾附近的注释掉的print语句）会使错误消失。

我知道即使遇到错误信息

• 该项目已在数据库中成功提交，
• 对项目执行GET显示URI完全正常

基于此，我认为问题可能在于POST return语句。我会非常感激地提供任何帮助 - 我不想忍受一个片状的解决方法！

端点

api.add_resource(BrandListResource, '/brands', endpoint = 'brands')
api.add_resource(BrandResource, '/brands/<string:brand_symbol>', endpoint = 'brand')

资源

brand_fields = {
    'id': fields.Integer,
    'brand_symbol': fields.String,
    'brand_name': fields.String,
    'uri': fields.Url('brand', absolute=True)
}

class BrandResource(Resource):
    @marshal_with(brand_fields)
    def get(self, brand_symbol):
        brand = db.session.query(Brand).filter(Brand.brand_symbol == brand_symbol).first()
        if not brand:
            abort(404, message="Brand {} doesn't exist".format(brand_symbol))
        return brand

class BrandListResource(Resource):
    @marshal_with(brand_fields)
    def get(self):
        brands = db.session.query(Brand).all()
        return brands

    @marshal_with(brand_fields)
    def post(self):
        parsed_args = parser.parse_args()
        brand_symbol = parsed_args['brand_symbol']
        brand_name = parsed_args['brand_name']

        brand = db.session.query(Brand).filter(Brand.brand_symbol == brand_symbol).first()
        if brand:
            abort(404, message="Brand {} already exists".format(brand_symbol))

        brand = Brand(brand_symbol=brand_symbol, brand_name=brand_name)
        db.session.add(brand)
        db.session.commit()

        #print brand
        return brand, 201

SQLAlchemy模型

class Brand(db.Model):
    __tablename__ = 'brand'

    id = db.Column(db.Integer, primary_key=True)
    brand_symbol = db.Column(db.String(5), unique=True)
    brand_name = db.Column(db.String(200), nullable=False)

    def __init__(self, brand_symbol, brand_name):
        self.brand_symbol = brand_symbol
        self.brand_name = brand_name

    def __repr__(self):
        return '<brand_symbol {}>'.format(self.brand_symbol)

完整错误回溯

Traceback (most recent call last):
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 2000, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1991, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 271, in error_router
    return original_handler(e)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1567, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 268, in error_router
    return self.handle_error(e)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 271, in error_router
    return original_handler(e)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 268, in error_router
    return self.handle_error(e)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 477, in wrapper
    resp = resource(*args, **kwargs)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/views.py", line 84, in view
    return self.dispatch_request(*args, **kwargs)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 587, in dispatch_request
    resp = meth(*args, **kwargs)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 682, in wrapper
    return marshal(data, self.fields, self.envelope), code, headers
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 640, in marshal
    return OrderedDict([(envelope, OrderedDict(items))]) if envelope else OrderedDict(items)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/collections.py", line 57, in __init__
    self.__update(*args, **kwds)
  File "/Users/skavie/testproject/lib/python2.7/_abcoll.py", line 571, in update
    for key, value in other:
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/__init__.py", line 639, in <genexpr>
    for k, v in fields.items())
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask_restful/fields.py", line 307, in output
    o = urlparse(url_for(endpoint, _external=self.absolute, **data))
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/helpers.py", line 332, in url_for
    return appctx.app.handle_url_build_error(error, endpoint, values)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/app.py", line 1811, in handle_url_build_error
    reraise(exc_type, exc_value, tb)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/flask/helpers.py", line 322, in url_for
    force_external=external)
  File "/Users/skavie/testproject/lib/python2.7/site-packages/werkzeug/routing.py", line 1758, in build
    raise BuildError(endpoint, values, method, self)
BuildError: Could not build url for endpoint 'brand' with values ['_sa_instance_state']. Did you forget to specify values ['brand_symbol']?

Answer 1

提交SQLAlchemy会话时，brand对象已过期。 stacktrace通过指出由于您要返回的对象中缺少某些信息而无法构建url来提供提示。

在这种情况下，brand对象需要刷新，因此它将再次包含所有数据。您可以通过打印它来做到这一点，但是像这样使用会话的refresh（）方法似乎要好一些：

...
db.session.commit()
db.session.refresh(brand)
return brand, 201

有一个小警告，但是，刷新方法的documentation在撰写本文时指出以下内容：

  

请注意，高度隔离的事务将返回与先前在同一事务中读取的值相同的值，而不管该事务外部的数据库状态如何变化-使用refresh（）通常仅在非ORM SQL语句被使用时才有意义。在正在进行的事务中发出，或者如果自动提交模式已打开。

这意味着您要刷新的信息可能已过时，但是我想在这种情况下这几乎是理论上的。