SQLAlchemy multiple foreign keys in one mapped class to the same primary key

Question

Am trying to setup a postgresql table that has two foreign keys that point to the same primary key in another table.

When I run the script I get the error

Answer 1

The latest documentation:

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

The form of foreign_keys= in the documentation produces a NameError, not sure how it is expected to work when the class hasn't been created yet. With some hacking I was able to succeed with this:

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')

In other words:

… foreign_keys='CurrentClass.thing_id')

Answer 2

Tried removing quotes from the foreign_keys and making them a list. From official documentation on Relationship Configuration: Handling Multiple Join Paths

Changed in version 0.8: relationship() can resolve ambiguity between foreign key targets on the basis of the foreign_keys argument alone; the primaryjoin argument is no longer needed in this situation.

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Self-contained code below works with 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()