SQLAlchemy將一個映射的類中的多個外鍵映射到同一主鍵

Question

我正在嘗試建立一個PostgreSQL表，該表具有兩個指向另一個表中相同主鍵的外鍵。

運行腳本時出現錯誤

sqlalchemy.exc.AmbiguousForeignKeysError：無法確定關系Company.stakeholder的父/子表之間的聯接條件-有多個鏈接表的外鍵路徑。 指定“ foreign_keys”參數，提供這些列的列表，這些列應被視為包含對父表的外鍵引用。

那是SQLAlchemy文檔中的確切錯誤，但是當我復制他們作為解決方案提供的內容時，該錯誤不會消失。 我可能做錯了什么？

#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')

我在這里看到了類似的問題，但一些問題的答案推薦一個使用primaryjoin但在文檔中就指出，你不需要primaryjoin在這種情況下。

Answer 1

試圖從foreign_keys中刪除引號並將它們制成列表。 來自有關Relationship Configuration: Handling Multiple Join Paths官方文檔Relationship Configuration: Handling Multiple Join Paths

在版本0.8中進行了更改： relationship()可以僅基於foreign_keys參數解決外鍵目標之間的歧義； 在這種情況下，不再需要primaryjoin參數。

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下面的自包含代碼適用於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

最新文檔：

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

文檔中foreign_keys=的形式會產生NameError，不確定在尚未創建類時預期如何工作。 通過一些黑客，我得以成功實現了這一點：

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')

換一種說法：

… foreign_keys='CurrentClass.thing_id')