从多对多关系中删除 SQLAlchemy

Question

从多对多关系中删除 SQLAlchemy

KA0*_*A01 7 python sqlite sqlalchemy flask flask-sqlalchemy

（我正在使用 SQLAlchemy、SQLite3、Flask-SQLAlchemy、Flask 和 Python）

我正在实施一个待办事项列表提要，用户可以在其中创建帖子 ( class Post) 并将任务 ( class Task)附加到每个帖子。每个任务可以有多个帖子。每个帖子可以有多个任务。我在使用 SQLAlchemy 并从表中删除时遇到问题。有趣的是：

当用户删除其中有零个帖子的任务( task.posts.count() == 0) 时，从数据库中删除成功
当用户删除包含一个或多个帖子的任务( task.posts.count() > 0) 时，从数据库中删除会引发错误。

这是错误：

sqlalchemy.exc.InvalidRequestError: This Session's transaction has been rolled back due to a previous exception during flush. 
To begin a new transaction with this Session, first issue Session.rollback().
Original exception was: DELETE statement on table 'tasks_posts' expected to delete 1 row(s); Only 0 were matched.

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这是 Post & Task Models & tasks_posts 表：

class Post(db.Model):
    __tablename__ = 'posts'
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.Text)
    tasks = db.relationship('Task', secondary='tasks_posts', \
            backref=db.backref('post', lazy='joined'), \
            lazy='dynamic', cascade='all, delete-orphan', \
            single_parent=True)
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))

class Task(db.Model):
    __tablename__ = 'tasks'
    id = db.Column(db.Integer, primary_key=True) 
    title = db.Column(db.String(24))
    description = db.Column(db.String(64))
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    posts = db.relationship('Post', secondary='tasks_posts', \
            backref=db.backref('task', lazy='joined'), \
            lazy='dynamic', cascade='all, delete-orphan', \
            single_parent=True)

tasks_posts = db.Table('tasks_posts',\
        db.Column('task_id', db.Integer, db.ForeignKey('tasks.id')),\
        db.Column('post_id', db.Integer, db.ForeignKey('posts.id'))\
        )

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这是视图函数：

@main.route('/edit-task/delete/<int:id>', methods=['GET', 'POST'])
def delete_task(id):
    task = Task.query.get_or_404(id)
    db.session.delete(task)
    db.session.commit()
    return redirect(url_for('.user', username=current_user.username))

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我假设问题是我错误地实施：

SQLAlchemy 的“级联”功能
多对多关系
或视图函数

这是堆栈跟踪：

File "...venv/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1473, in full_dispatch_request
    rv = self.preprocess_request()
  File ".../venv/lib/python2.7/site-packages/flask/app.py", line 1666, in preprocess_request
    rv = func()
  File ".../app/auth/views.py", line 12, in before_request
    if current_user.is_authenticated:
  File ".../venv/lib/python2.7/site-packages/werkzeug/local.py", line 342, in __getattr__
    return getattr(self._get_current_object(), name)
  File ".../venv/lib/python2.7/site-packages/werkzeug/local.py", line 301, in _get_current_object
    return self.__local()
  File ".../venv/lib/python2.7/site-packages/flask_login.py", line 47, in <lambda>
    current_user = LocalProxy(lambda: _get_user())
  File ".../venv/lib/python2.7/site-packages/flask_login.py", line 858, in _get_user
    current_app.login_manager._load_user()
  File ".../venv/lib/python2.7/site-packages/flask_login.py", line 389, in _load_user
    return self.reload_user()
  File ".../venv/lib/python2.7/site-packages/flask_login.py", line 351, in reload_user
    user = self.user_callback(user_id)
  File ".../app/models.py", line 235, in load_user
    return User.query.get(int(user_id))
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 829, in get
    return self._get_impl(ident, loading.load_on_ident)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 853, in _get_impl
    self.session, key, attributes.PASSIVE_OFF)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 152, in get_from_identity
    state._load_expired(state, passive)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/state.py", line 474, in _load_expired
    self.manager.deferred_scalar_loader(self, toload)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 664, in load_scalar_attributes
    only_load_props=attribute_names)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 219, in load_on_ident
    return q.one()
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2528, in one
    ret = list(self)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2571, in __iter__
    return self._execute_and_instances(context)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2584, in _execute_and_instances
    close_with_result=True)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2575, in _connection_from_session
    **kw)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 893, in connection
    execution_options=execution_options)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 898, in _connection_for_bind
    engine, execution_options)
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 313, in _connection_for_bind
    self._assert_active()
  File ".../venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 214, in _assert_active
    % self._rollback_exception
InvalidRequestError: This Session's transaction has been rolled back due to a previous exception during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original exception was: DELETE statement on table 'tasks_posts' expected to delete 1 row(s); Only 0 were matched.

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Answer 1

pec*_*rin 1

好的，所以我认为这里发生的一些事情可能会导致您的问题。首先是错误消息本身。这意味着数据库认为它应该删除某些内容，但它不在那里。我相信这是由你delete-all orphan和single_parent=True引起的。

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这告诉 sqlalchemyPost和Task一个 single_parent，这令人困惑！所以我相信你需要做的就是让它发挥作用

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仅在一个模型上定义关系。现在使用两个定义关系的类来设置它的方式正在使您的代码混乱。我会建议这样的事情：

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class Post(db.Model):\n    __tablename__ = \'posts\'\n    id = db.Column(db.Integer, primary_key=True)\n    body = db.Column(db.Text)\n    tasks = db.relationship(\'Task\', secondary=\'tasks_posts\', \\\n                            backref=db.backref(\'post\', lazy=\'joined\'), \\\n                            lazy=\'dynamic\', cascade=\'all, delete-orphan\', \\\n                            single_parent=True)\n    user_id = db.Column(db.Integer, db.ForeignKey(\'users.id\'))\n\nclass Task(db.Model):\n    __tablename__ = \'tasks\'\n    id = db.Column(db.Integer, primary_key=True)\n    title = db.Column(db.String(24))\n    description = db.Column(db.String(64))\n    user_id = db.Column(db.Integer, db.ForeignKey(\'users.id\'))\n

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弄清楚您希望数据模型如何工作。任何任务可以在任何帖子中，并且任何帖子可以有任意数量的任务？我认为您也许应该通过发布自己的任务来重新考虑数据模型。您仍然可以在不同的两者中共享任务，但您需要清楚地了解未来的数据模型。
明确您要删除的内容。我知道，当删除任务时，任务所在的每个帖子都应该被删除，这对您来说可能是有意义的，但对我来说，这没有意义。循环浏览要删除的正确帖子和任务。这样你会对删除和更干净的代码有更好的理解。

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更新：

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从文档中中：

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这里有几种可能性：
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如果存在从父级到子级的关系（），但没有将特定子级链接到每个父级的反向关系，\n SQLAlchemy 将不会意识到在删除此\n 特定子级对象时，它需要维护将其链接到父级的 \xe2\x80\x9csecondary\xe2\x80\x9d 表\n。不会删除 \xe2\x80\x9csecondary\xe2\x80\x9d 表。\n
\n
如果存在将特定 Child 链接到每个 Parent 的关系，假设它\xe2\x80\x99 称为 Child.parents，则 SQLAlchemy 默认情况下将\n 加载到 Child.parents 集合中以查找所有 Parent 对象，并\n 删除建立此链接的 \xe2\x80\x9csecondary\xe2\x80\x9d 表中的每一行。请注意，此关系不需要是双向关系；\n SQLAlchemy 严格查看与要删除的子对象关联的每个关系()。
\n
这里性能更高的选项是将 ON DELETE CASCADE 指令与数据库使用的外键一起使用。假设数据库支持此功能，则可以让数据库本身自动删除 \xe2\x80\x9csecondary\xe2\x80\x9d 表中的行，以引用 \xe2\x80\x9cchild\xe2 中的行\n \x80\x9d 被删除。在这种情况下，可以使用关系（）上的passive_deletes指令指示SQLAlchemy放弃在Child.parents集合中主动加载；有关详细信息，请参阅使用被动\n 删除。再次注意，这些行为仅与关系（）使用的辅助选项相关。如果处理\n 显式映射且不存在\n 相关关系() 的辅助选项\n 中的关联表，则可以使用级联规则\n 来自动删除实体，以响应删除的相关实体\n - 有关此功能的信息，请参阅级联。\n
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