python - SQLAlchemy声明性mixin类上的自引用外键关系与declared_attr

Question

我有一个 mixin 类，我在 SQLAlchemy 应用程序的开头附近定义了它，然后继承了我使用的几乎所有声明性模型。

class Owned(object):

    @declared_attr
    def created_by_id(cls):
        return Column(Integer, ForeignKey("accounts.id"),
            nullable = True)

    @declared_attr
    def created_by(cls):
        return relationship("Account", foreign_keys = cls.created_by_id)

    @declared_attr
    def updated_by_id(cls):
        return Column(Integer, ForeignKey("accounts.id"),
            nullable = True)

    @declared_attr
    def updated_by(cls):
        return relationship("Account", foreign_keys = cls.updated_by_id)

这适用于大多数预期的用例。

class Thing(Owned, Base): # Base is from SQLAlchemy's declarative_base()
    pass

account = session.query(Account).first()

thing = Thing(created_by = account, updated_by = account)

session.add(thing)
session.commit()
session.refresh(thing)

assert thing.created_by == account # pass
assert thing.updated_by == account # pass

但是，当我将Account自己定义为继承自Owned.

class Account(Owned, Base):
    pass

account_old = session.query(Account).first()

account_new = Account(created_by = account_old, updated_by = account_old)

session.add(account_new)
session.commit()
session.refresh(account_new)

assert account_new.created_by_id == account_old.id # pass
assert account_new.updated_by_id == account_old.id # pass

# BUT!

assert account_new.created_by == account_old # fail
assert account_new.updated_by == account_old # fail

account_new.created_by # []
account_new.updated_by # []

我看到，在这种情况下，我已经变成created_by_id了updated_by_id自引用外键。然而，我不明白为什么 SQLAlchemy 没有用预期的实例填充它们的关联relationship列。Account

我究竟做错了什么？

标签： pythonsqlalchemyself-referencing-table

Answer 1

在邻接列表关系中，“方向”默认为一对多。使用该remote_side指令确定关系是多对一的，这就是您所追求的：

def _create_relationship(cls, foreign_keys):
    kwgs = {}
    # Bit of a chicken or egg situation:
    if cls.__name__ == "Account":
        kwgs["remote_side"] = [cls.id]

    return relationship("Account", foreign_keys=foreign_keys, **kwgs)


class Owned:

    @declared_attr
    def created_by_id(cls):
        return Column(Integer, ForeignKey("accounts.id"),
            nullable = True)

    @declared_attr
    def created_by(cls):
        return _create_relationship(cls, cls.created_by_id)

    @declared_attr
    def updated_by_id(cls):
        return Column(Integer, ForeignKey("accounts.id"),
            nullable = True)

    @declared_attr
    def updated_by(cls):
        return _create_relationship(cls, cls.updated_by_id)