Flask SQLAlchemy 多个连接路径和一对多关系答案

【问题标题】：Flask SQL Alchemy Multiple join paths and one to many relationshipsFlask SQLAlchemy 多个连接路径和一对多关系
【发布时间】：2018-08-01 08:54:30
【问题描述】：

我正在尝试在 Flask 中创建任务管理应用程序并遇到以下问题。

我有 3 个模型，User、Task 和 Project。我希望用户能够创建任务并将它们分配给其他用户。这需要在 Task 和 User 模型之间进行多个连接。

以前，在尝试实现此功能之前，我有一个从 User 到 Task 的一对多关系，这工作正常，下面是两个原始模型。

class User(UserMixin, db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=True )
    email = db.Column(db.String(120), unique=True, nullable=True)
    password_hash = db.Column(db.String(128))

    tasks = db.relationship('Task', backref='author', lazy='dynamic') # one user has many tasks
    projects = db.relationship('Project', backref='author', lazy='dynamic') # one user has many projects

class Task(UserMixin, db.Model):
    __tablename__ = 'tasks'
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.String(128))
    due = db.Column(db.DateTime, index=True)
    done = db.Column(db.Boolean, default=False)
    author_id = db.Column(db.Integer, db.ForeignKey('users.id')) # task belongs to user
    project_id = db.Column(db.Integer, db.ForeignKey('projects.id')) # task belongs to project

在SQL Alchemy documentation 之后，我在Task 中添加了assigned_to_id 列，在User 中添加了assigned_tasks 列。

我的模型现在如下：

class User(UserMixin, db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=True )
    email = db.Column(db.String(120), unique=True, nullable=True)
    password_hash = db.Column(db.String(128))
    tasks_id = db.Column(db.Integer, db.ForeignKey('tasks.id'))
    assigned_tasks_id = db.Column(db.Integer, db.ForeignKey('tasks.id'))

    tasks = db.relationship('Task', foreign_keys="User.tasks_id", backref='author') # one user has many tasks
    assigned_tasks = db.relationship('Task', foreign_keys="User.tasks_id", backref='assigned_to') # one user has many tasks
    projects = db.relationship('Project', backref='author', lazy='dynamic') # one user has many projects

class Task(UserMixin, db.Model):
    __tablename__ = 'tasks'
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.String(128))
    due = db.Column(db.DateTime, index=True)
    done = db.Column(db.Boolean, default=False)
    author_id = db.Column(db.Integer, db.ForeignKey('users.id')) # task belongs to user
    assigned_to_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    project_id = db.Column(db.Integer, db.ForeignKey('projects.id')) # task belongs to project

由于我对需要分配的任务或需要分配任务的用户没有任何限制，我假设我应该能够像以前一样继续创建用户和任务。但是现在，当我尝试与作者一起创建任务时，出现错误。

>>> u = User(email='test@example.com', password='password')
>>> t = Task(body='New Task', author=u)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "<string>", line 4, in __init__
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 417, in _initialize_instance
    manager.dispatch.init_failure(self, args, kwargs)
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 249, in reraise
    raise value
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 414, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "/Users/Jasmine/projects/flask/tasky/app/models.py", line 67, in __init__
    super(Task, self).__init__(**kwargs)
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/ext/declarative/base.py", line 700, in _declarative_constructor
    setattr(self, k, kwargs[k])
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 229, in __set__
    instance_dict(instance), value, None)
  File "/Users/Jasmine/projects/flask/tasky/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 1040, in set
    given, wanted))
TypeError: Incompatible collection type: User is not list-like

我可能误解了这里的错误，但在我看来，现在每个任务都需要不止一个作者。如何保持一对多关系，以便每个任务仍然只需要一个作者？

【问题讨论】：

标签： python-3.x flask flask-sqlalchemy

【解决方案1】：

由于我的声誉，我无法添加评论，但是 One-to-many Flask | SQLAlchemy

此处接受的答案应该对您有所帮助。还有……

>>> u = User(email='test@example.com', password='password')
>>> t = Task(body='New Task', author=u)

Task 类中不存在 author 变量，因为它存在于 User 类中

您需要将用户附加到任务。您可能想查看您的代码，类之间的关系似乎不正确。否则我错了。

【讨论】：