SQLAlchemy将一个映射的类中的多个外键映射到同一主键

Question

我正在尝试建立一个PostgreSQL表，该表具有两个指向另一个表中相同主键的外键。

运行脚本时出现错误

sqlalchemy.exc.AmbiguousForeignKeysError：无法确定关系Company.stakeholder的父/子表之间的联接条件-有多个链接表的外键路径。 指定“ foreign_keys”参数，提供这些列的列表，这些列应被视为包含对父表的外键引用。

那是SQLAlchemy文档中的确切错误，但是当我复制他们作为解决方案提供的内容时，该错误不会消失。 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')

我在这里看到了类似的问题，但一些问题的答案推荐一个使用primaryjoin但在文档中就指出，你不需要primaryjoin在这种情况下。

Answer 1

试图从foreign_keys中删除引号并将它们制成列表。 来自有关Relationship Configuration: Handling Multiple Join Paths官方文档Relationship Configuration: Handling Multiple Join Paths

在版本0.8中进行了更改： relationship()可以仅基于foreign_keys参数解决外键目标之间的歧义； 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NameError，不确定在尚未创建类时预期如何工作。 通过一些黑客，我得以成功实现了这一点：

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')

换一种说法：

… foreign_keys='CurrentClass.thing_id')