Python SqlAlchemy - AttributeError：映射器

Question

基于我的模型：

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship

Base = declarative_base()

class Session(Base):
    __tablename__ = 'sessions'

    id = Column(Integer, primary_key=True)
    token = Column(String(200))
    user_id = Column(Integer, ForeignKey('app_users.id'))
    user = relationship('model.user.User', back_populates='sessions')

我想通过以下方式实例化一个新会话：

session = Session(token='test-token-123')

但我明白了：

AttributeError: mapper

完整的堆栈跟踪：

Traceback (most recent call last):
  File "/home/ubuntu/.local/lib/python3.5/site-packages/falcon/api.py", line 227, in __call__
    responder(req, resp, **params)
  File "./app_user/register.py", line 13, in on_post
    session = Session(token='test-token-123')
  File "<string>", line 2, in __init__
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/instrumentation.py", line 347, in _new_state_if_none
    state = self._state_constructor(instance, self)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 764, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/instrumentation.py", line 177, in _state_constructor
    self.dispatch.first_init(self, self.class_)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/event/attr.py", line 256, in __call__
    fn(*args, **kw)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py", line 2976, in _event_on_first_init
    configure_mappers()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py", line 2872, in configure_mappers
    mapper._post_configure_properties()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py", line 1765, in _post_configure_properties
    prop.init()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/interfaces.py", line 184, in init
    self.do_init()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1653, in do_init
    self._process_dependent_arguments()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1710, in _process_dependent_arguments
    self.target = self.mapper.mapped_table
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 850, in __getattr__
    return self._fallback_getattr(key)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 828, in _fallback_getattr
    raise AttributeError(key)

我不知道这个错误是从哪里来的，我也无法真正调试它。任何人都可以帮助我解决这个问题吗？

谢谢和问候！

Answer 1

查看回溯，您可以看到以下几行：

  ...
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1653, in do_init
    self._process_dependent_arguments()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1710, in _process_dependent_arguments
    self.target = self.mapper.mapped_table
  ...

这大大缩小了您的问题。关系

    user = relationship('model.user.User', back_populates='sessions')

使用 Python 可评估字符串作为argument，"Configuring Relationships" 中进一步解释了其用法：

与其他类的关系以通常的方式完成，增加了relationship() 指定的类可能是字符串名称的特性。与Base 关联的“类注册表”在映射器编译时用于将名称解析为实际的类对象，预计在使用映射器配置后已定义

如果您在第一次尝试实例化 Session 对象之前没有在任何地方导入 models.user 模块，那么名称解析将失败，因为尚未创建类 User 并且不存在于注册表。换句话说，要使名称解析起作用，必须已定义所有类，这意味着它们的主体必须已被执行。

如果您确实导入了models.user 模块，请检查您的其他模型，并确保它们的相关模型类已定义。第一次使用您的模型会触发映射器编译/配置，因此错误的来源也可能是其他模型。