SQLAlchemy中实体双外键指向同一实体时的无关联外键配置问题

报错原因是UserEntity.last_message关系未明确指定关联的外键，当MessageEntity存在两个指向user表的外键（from_user_id和to_user_id）时，SQLAlchemy无法自动推断应该用哪条外键建立这条一对一关系。

要实现to_user_id仅作为ID存储而不建立ORM关系，只需在UserEntity的last_message关系中补充foreign_keys参数，明确指定使用from_user_id即可：

from sqlalchemy import ForeignKey
from sqlalchemy.orm import mapped_column, relationship, Mapped, DeclarativeBase

class Base(DeclarativeBase):
    pass

class UserEntity(Base):
    __tablename__ = "user"

    id: Mapped[str] = mapped_column(primary_key=True)
    # 补充foreign_keys参数，指定关联from_user_id
    last_message = relationship(
        "MessageEntity", 
        back_populates="from_user", 
        uselist=False,
        foreign_keys="[MessageEntity.from_user_id]"
    )

class MessageEntity(Base):
    __tablename__ = "message"

    id: Mapped[str] = mapped_column(primary_key=True)
    content: Mapped[str] = mapped_column()
    from_user_id: Mapped[str] = mapped_column(ForeignKey('user.id'))
    # to_user_id仅作为外键字段存储，不建立ORM关系
    to_user_id: Mapped[str] = mapped_column(ForeignKey('user.id'))
    # relationships
    from_user = relationship(
        "UserEntity", 
        back_populates="last_message", 
        foreign_keys=[from_user_id], 
        uselist=False
    )

修改后，to_user_id会作为普通外键字段存在于数据库表中，仅存储用户ID，不会和UserEntity建立额外ORM关系，同时SQLAlchemy能明确UserEntity.last_message与MessageEntity.from_user的关联路径，解决歧义报错。

内容的提问来源于stack exchange，提问作者kuco 23