Question

我有一个像这样定义的模型类。我们的想法是，Person将包含有关人员的一般信息，并提及它的子类和＃34;：女人和男人作为父母。女人和男人将持有针对该性别的具体信息。

我使用Alembic生成迁移但我收到错误：

sqlalchemy.exc.AmbiguousForeignKeysError：无法确定＆＃39; person＆＃39;之间的联接和女人＆＃39 ;;表之间有多个外键约束关系。请指明＆＃39; onclause＆＃39;明确加入。

请注意，女性继承自人，因此也存在女性对人的引用。

我没有尝试过任何作品：

删除行mother = relationship('Woman')，认为SQL炼金术不理解这种关系
添加mother_id = Column(Integer, ForeignKey('woman.id'), nullable=True)
添加mother = relationship('Woman', primaryjoin='Person.mother_id==Mother.id')

指定onclause是不可能的（或者我不知道怎么做），因为连接是自动生成的

class Person(Base):
    __tablename__ = 'person'

    id = Column(Integer, primary_key=True)
    gender = Column(String(5))
    name = Column(String(32), nullable=False)
    surname = Column(String(32), nullable=False)
    age = Column(Float, primary_key=True)

    mother_id = Column(Integer, ForeignKey('woman.id'), nullable=True)
    mother = relationship('Woman')

    _mapper_args__ = {
        'polymorphic_on': gender
    }


class Woman(Person):
    __tablename__ = 'woman'
    __mapper_args__ = {
        'polymorphic_identity': 'woman'
    }

    id = Column(Integer, ForeignKey('person.id'), primary_key=True)

完整追踪：

$ alembic revision --autogenerate -m "mother"
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.4/bin/alembic", line 11, in <module>
    sys.exit(main())
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/config.py", line 479, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/config.py", line 473, in main
    self.run_cmd(cfg, options)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/config.py", line 456, in run_cmd
    **dict((k, getattr(options, k)) for k in kwarg)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/command.py", line 117, in revision
    script_directory.run_env()
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/script/base.py", line 416, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/util/pyfiles.py", line 93, in load_python_file
    module = load_module_py(module_id, path)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/alembic/util/compat.py", line 68, in load_module_py
    module_id, path).load_module(module_id)
  File "<frozen importlib._bootstrap>", line 539, in _check_name_wrapper
  File "<frozen importlib._bootstrap>", line 1614, in load_module
  File "<frozen importlib._bootstrap>", line 596, in _load_module_shim
  File "<frozen importlib._bootstrap>", line 1220, in load
  File "<frozen importlib._bootstrap>", line 1200, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1129, in _exec
  File "<frozen importlib._bootstrap>", line 1471, in exec_module
  File "<frozen importlib._bootstrap>", line 321, in _call_with_frames_removed
  File "migrations/env.py", line 24, in <module>
    import api.model
  File "/Users/jakubt/Projects/hex/api/model.py", line 31, in <module>
    class Woman(Person):
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/ext/declarative/api.py", line 64, in __init__
    _as_declarative(cls, classname, cls.__dict__)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 88, in _as_declarative
    _MapperConfig.setup_mapping(cls, classname, dict_)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 103, in setup_mapping
    cfg_cls(cls_, classname, dict_)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 135, in __init__
    self._early_mapping()
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 138, in _early_mapping
    self.map()
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 530, in map
    **self.mapper_args
  File "<string>", line 2, in mapper
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/orm/mapper.py", line 671, in __init__
    self._configure_inheritance()
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/orm/mapper.py", line 978, in _configure_inheritance
    self.local_table)
  File "<string>", line 2, in join_condition
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/sql/selectable.py", line 965, in _join_condition
    a, b, constraints, consider_as_foreign_keys)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sqlalchemy/sql/selectable.py", line 1055, in _joincond_trim_constraints
    "join explicitly." % (a.description, b.description))
sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between 'person' and 'woman'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly.

Answer 1

我在这里看到的问题是你将外键引用到外键。

mother_id -> foreign_key(woman.id)
woman.id -> foreign_key(person.id)

并创建循环依赖。您无法在任何SQL方言的任何单个创建语句中创建该依赖项。您首先需要创建person表，然后创建woman表，然后从第一个表到第二个表添加foreign key (mother_id)（请注意，您不能先创建女性表，因为它有人员表的外键）。

我不认为alembic足够聪明，而且我认为你想要实现的目标并不是一件好事。

以下是我的建议：

woman.id对于一条记录总是与person.id具有相同的值，因此第一个外键应该引用person表而不是母表。

更改mother_id和mother的定义：

mother_id = Column(Integer, ForeignKey('person.id'), nullable=True)
mother = relationship('Person', primaryjoin='Person.mother_id==Person.id')

提示：如果您只是存储id（只需从第二课中删除__tablename__），则您不需要第二个表格

在添加母亲/父亲时检查代码中的性别。

自引用表中不明确的外键[SQLAlchemy / Alembic]

1 个答案: