sqlalchemy + flask: class 未定义

Question

我正在使用 sqlalchemy + alembic + Flask，我无法映射循环类。

apps/users/models.py:

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    email = Column(String)
    password = Column(String)
    session = relationship("Session", back_populates='user', cascade='all,delete', lazy='dynamic')

    notes = relationship('Note2User', back_populates='user', cascade='all,delete', lazy='dynamic')

apps/notes/models.py:

class Note2User(Base):
    __tablename__ = 'notes_users_m2m'

    id = Column(Integer, primary_key=True)

    user_id = Column(Integer, ForeignKey('users.id', ondelete='CASCADE'), nullable=False)
    user = relationship('User', back_populates='notes')
    note_id = Column(Integer, ForeignKey('notes.id', ondelete='CASCADE'), nullable=False)
    note = relationship('Note', back_populates='users')

Table Note2User 为 m2m 关系用户 <-> 注释，但是当我启动应用程序并完成一些请求时，出现错误：

InvalidRequestError: When initializing mapper Mapper|User|users, expression 'Note2User' failed to locate a name ("name 'Note2User' is not defined"). If this is a class name, consider adding this relationship() to the class after both dependent classes have been defined.

正在 db/init.py 中初始化 db: (dunder name)

from sqlalchemy import create_engine, MetaData
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session

engine = create_engine('postgresql+psycopg2://server:12345@localhost:5432/test')

Base = declarative_base()

meta = MetaData()
meta.reflect(bind=engine)

db_session = Session(bind=engine)

Answer 1

在 apps/users/models.py 文件中添加 Note2User class 的导入，以便在 User class 中初始化该关系之前首先定义此模型，从而引用它. 像这样

# file: apps/users/models.py
from ..notes.models import Note2User

Answer 2

您需要将 user.models 模块导入 notes.model 模块，反之亦然。它看起来像这样：

# file app/users/models.py
import app.notes.models as notes
# use it like this
notes.Notes2User()


# file app/notes/models.py
import app.users.models as users
users.User()

这样做的好处是，当程序不可避免地增长时，您将避免循环依赖问题。当我使用相同的堆栈创建应用程序时，我遇到了很多循环依赖问题。唯一的解决办法是放弃

from . import Foo

并且只使用

import bar.foo as foo

出于这个原因，使用导入语法被认为是最佳做法。 Reference.

sqlalchemy + flask: class 未定义

sqlalchemy + flask: class is not defined

python

flask

sqlalchemy

alembic