SQLAlchemy 一个映射类中的多个外键到同一个主键

Question

我正在尝试设置一个 postgresql 表，该表具有两个指向另一个表中相同主键的外键。

当我运行脚本时出现错误

sqlalchemy.exc.AmbiguousForeignKeysError：无法确定关系 Company.stakeholder 上的父/子表之间的连接条件 - 有多个外键路径链接表。指定 'foreign_keys' 参数，提供应计为包含对父表的外键引用的列的列表。

这是SQLAlchemy Documentation 中的确切错误，但当我复制他们提供的解决方案时，错误并没有消失。我可能做错了什么？

#The business case here is that a company can be a stakeholder in another company.
class Company(Base):
    __tablename__ = 'company'
    id = Column(Integer, primary_key=True)
    name = Column(String(50), nullable=False)

class Stakeholder(Base):
    __tablename__ = 'stakeholder'
    id = Column(Integer, primary_key=True)
    company_id = Column(Integer, ForeignKey('company.id'), nullable=False)
    stakeholder_id = Column(Integer, ForeignKey('company.id'), nullable=False)
    company = relationship("Company", foreign_keys='company_id')
    stakeholder = relationship("Company", foreign_keys='stakeholder_id')

我在这里看到过类似的问题，但some of the answers 建议使用primaryjoin，但在文档中它指出在这种情况下您不需要primaryjoin。

Answer 1

尝试从 foreign_keys 中删除引号并使其成为一个列表。来自Relationship Configuration: Handling Multiple Join Paths的官方文档

0.8 版更改：relationship() 可以解决之间的歧义 仅基于 foreign_keys 参数的外键目标； 在这种情况下，不再需要 primaryjoin 参数。

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下面的自包含代码适用于sqlalchemy>=0.9：

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

engine = create_engine(u'sqlite:///:memory:', echo=True)
session = scoped_session(sessionmaker(bind=engine))
Base = declarative_base()

#The business case here is that a company can be a stakeholder in another company.
class Company(Base):
    __tablename__ = 'company'
    id = Column(Integer, primary_key=True)
    name = Column(String(50), nullable=False)

class Stakeholder(Base):
    __tablename__ = 'stakeholder'
    id = Column(Integer, primary_key=True)
    company_id = Column(Integer, ForeignKey('company.id'), nullable=False)
    stakeholder_id = Column(Integer, ForeignKey('company.id'), nullable=False)
    company = relationship("Company", foreign_keys=[company_id])
    stakeholder = relationship("Company", foreign_keys=[stakeholder_id])

Base.metadata.create_all(engine)

# simple query test
q1 = session.query(Company).all()
q2 = session.query(Stakeholder).all()

Answer 2

最新文档：

• http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#handling-multiple-join-paths

文档中foreign_keys= 的形式会产生一个NameError，不确定在尚未创建类时它应该如何工作。通过一些黑客攻击，我能够成功：

company_id = Column(Integer, ForeignKey('company.id'), nullable=False)
company = relationship("Company", foreign_keys='Stakeholder.company_id')

stakeholder_id = Column(Integer, ForeignKey('company.id'), nullable=False)
stakeholder = relationship("Company",
                            foreign_keys='Stakeholder.stakeholder_id')

换句话说：

… foreign_keys='CurrentClass.thing_id')