python - 通过 Flask-SQL Alchemy 创建关联对象在 shell 中按预期工作，但在 Flask 路由中产生 JSON 错误

Question

我正在尝试创建一个关联对象，该对象定义和UserRelationship之间的“跟随”关系。当我通过 shell 与这个对象交互时，它的行为与我预期的一样，但是当实现到路由函数时，它会产生一个“TypeError：UserRelationship 类型的对象不是 JSON 可序列化的”UserUser

class UserRelationship(db.Model):
    follows_id = db.Column(db.Integer, db.ForeignKey('user.id'), primary_key=True)
    bank = db.Column(db.String(100))
    follows = db.relationship('User', backref=db.backref('followers'))

class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)
    follows = db.relationship('UserRelationship', backref=db.backref('followers', lazy=True))

这会在 shell 中产生预期的结果：

user1=User(username='joe')
user2=User(username='paul')
user3=User(username='phil')

new_follow = UserRelationship(follows=user2, bank='1000')
new_follow2 = UserRelationship(follows=user3, bank='1000') 

user2.follows.append(new_follow)
user2.follows.append(new_follow2)
db.session.commit()

user2.follows
>>[<UserRelationship (transient 2627038264968)>, <UserRelationship (transient 2627038286344)>]


user2.follows[0].bank
'1000'

user2.follows[0].follows
<User Paul>

但是，当我在我的 Flask 应用程序上实际使用它时，它会运行一个错误：

`TypeError：UserRelationship 类型的对象不是 JSON 可序列化的'

这是我尝试在实际应用程序上实现的方式：

@app.route('/follow/<to_follow>', methods=['GET', 'POST'])
@login_required
def follow(to_follow):
    follow_target = User.query.filter_by(username=to_follow).first()
    form = FollowForm()
    if form.validate_on_submit():
        new_follow = UserRelationship(bank='1000', follows=follow_target)
        current_user.follows.append(new_follow)
        db.session.commit()
        flash(current_user.follows)
        return redirect(profile)

    return render_template('follow.html',
                           title=f'Follow {to_follow}',
                           form=form,
                           to_follow=follow_target)

完整追溯：

Traceback (most recent call last):
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 1952, in full_dispatch_request
    return self.finalize_request(rv)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 1969, in finalize_request
    response = self.process_response(response)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 2268, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\sessions.py", line 378, in save_session
    val = self.get_signing_serializer(app).dumps(dict(session))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\serializer.py", line 166, in dumps
    payload = want_bytes(self.dump_payload(obj))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\url_safe.py", line 42, in dump_payload
    json = super(URLSafeSerializerMixin, self).dump_payload(obj)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\serializer.py", line 133, in dump_payload
    return want_bytes(self.serializer.dumps(obj, **self.serializer_kwargs))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\tag.py", line 305, in dumps
    return dumps(self.tag(value), separators=(",", ":"))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\__init__.py", line 211, in dumps
    rv = _json.dumps(obj, **kwargs)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\__init__.py", line 100, in default
    return _json.JSONEncoder.default(self, o)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type UserRelationship is not JSON serializable
[2019-09-29 16:10:38,924] ERROR in app: Request finalizing failed with an error while handling an error
Traceback (most recent call last):
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 1952, in full_dispatch_request
    return self.finalize_request(rv)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 1969, in finalize_request
    response = self.process_response(response)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 2268, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\sessions.py", line 378, in save_session
    val = self.get_signing_serializer(app).dumps(dict(session))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\serializer.py", line 166, in dumps
    payload = want_bytes(self.dump_payload(obj))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\url_safe.py", line 42, in dump_payload
    json = super(URLSafeSerializerMixin, self).dump_payload(obj)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\serializer.py", line 133, in dump_payload
    return want_bytes(self.serializer.dumps(obj, **self.serializer_kwargs))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\tag.py", line 305, in dumps
    return dumps(self.tag(value), separators=(",", ":"))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\__init__.py", line 211, in dumps
    rv = _json.dumps(obj, **kwargs)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\__init__.py", line 100, in default
    return _json.JSONEncoder.default(self, o)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type UserRelationship is not JSON serializable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 1969, in finalize_request
    response = self.process_response(response)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\app.py", line 2268, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\sessions.py", line 378, in save_session
    val = self.get_signing_serializer(app).dumps(dict(session))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\serializer.py", line 166, in dumps
    payload = want_bytes(self.dump_payload(obj))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\url_safe.py", line 42, in dump_payload
    json = super(URLSafeSerializerMixin, self).dump_payload(obj)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\itsdangerous\serializer.py", line 133, in dump_payload
    return want_bytes(self.serializer.dumps(obj, **self.serializer_kwargs))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\tag.py", line 305, in dumps
    return dumps(self.tag(value), separators=(",", ":"))
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\__init__.py", line 211, in dumps
    rv = _json.dumps(obj, **kwargs)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\site-packages\flask\json\__init__.py", line 100, in default
    return _json.JSONEncoder.default(self, o)
  File "c:\users\phill\anaconda3\envs\zigenv\lib\json\encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type UserRelationship is not JSON serializable

标签： pythonflasksqlalchemymany-to-many

Answer 1

实际上，db.Model该类没有提供以 JSON 格式序列化其实例的方法。例如，从Flask-SqlAlchemy 快速入门中提取的以下代码由于相同的原因而失败。

import json

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
db = SQLAlchemy(app)


class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)


if __name__ == '__main__':
    db.create_all()
    admin = User(username='admin', email='[e-mail]')

    json.dumps(admin)

你得到：

Traceback (most recent call last):
  [...]
TypeError: Object of type User is not JSON serializable

因此，您UserRelationship继承的类db.Model不是 JSON 可序列化的。

当服务器想要保存当前会话中的用户状态时，就会出现问题。它转储User实例，然后转储它的UserRelationship.

必须有一种方法可以避免将关系存储在会话中。但这需要更多的调查。

编辑

IMO，UserRelationship应该是a和它自身之间的关联对象。User这是一种带有额外列的自引用多对多关系。

基于这个this answer，你应该这样定义它：

class UserRelationship(db.Model):
    fk_user_from = db.Column(db.Integer, db.ForeignKey("user.id"), primary_key=True)
    fk_user_to = db.Column(db.Integer, db.ForeignKey("user.id"), primary_key=True)
    bank = db.Column(db.String(100))


class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    followings = db.relationship(
        "UserRelationship", backref="followed", primaryjoin=id == UserRelationship.fk_user_to
    )
    followers = db.relationship(
        "UserRelationship", backref="following", primaryjoin=id == UserRelationship.fk_user_from
    )

你能用这个模式重试吗？