与 SQLalchemy 中不同表的关系答案

【问题标题】：Relationship to different tables in SQLalchemy与 SQLalchemy 中不同表的关系
【发布时间】：2015-12-10 19:11:37
【问题描述】：

我有多个表。所有表都有多个列，这些列在逻辑上无法存储在一个合并表中。

class Foo(Model):
    foo_id = Column(Integer(unsigned=True), primary_key=True, nullable=False)
    foo_data = Column(...)

class Bar(Model):
    bar_id = Column(Integer(unsigned=True), primary_key=True, nullable=False)
    bar_info = Column(...)

class Baz(Model):
    baz_id = Column(Integer(unsigned=True), primary_key=True, nullable=False)
    baz_content = Column(...)

现在我想在历史表中保存更改：

class Diff(Model):
    diff_id = Column(...)
    foreign_id = Column(Integer(unsigned=True), index=True, nullable=False)
    foreign_table = Column(Char(16, collation='ascii_bin'), index=True, nullable=False)

class SingleDiff(Model):
     ...
     diff = relationship(Diff, backref=backref('changes', uselist=True, ...))

现在我的问题是：如何在插入新的Foo、Bar 或Baz 时在同一个提交中填充Diff.foreign_id？

这可行，但如果插入Diff 时出现问题，那么将更改回滚到foo 为时已晚：

foo = Foo(foo_data='TODO: changes table names')
try:
    session.add(foo)
    session.commit()  # first commit
except ...:
    ....
else:
    try:
         diff = Diff(changes=[...])
         diff.foreign_id = foo.foo_id
         diff.foerein_table = 'foo'
         session.add(diff)
         session.commit()  # second commit
     except ...:
         raise Exception('Cannot rollback commit #1 anymore :-(')

对于正常的关系，id 会自动插入：

class FooDiff(Model):
    diff_id = Column(...)
    foo_id = Column(Integer(unsigned=True), ForeignKey(...), ...)
    foo = relationship(Foo)

但我没有Diff.foo，因为Diff.foreign_id 可以指向许多不同的表。

【问题讨论】：

阅读文档的Generic Associations 部分。我认为尤其是Generic Foreign Key 示例应该适合您的情况。
@van，谢谢你的链接！

标签： python sqlalchemy foreign-keys one-to-many

【解决方案1】：

我想出的解决方案与@van所链接的几乎相同：ORM Examples: Generic Associations。

def _foreign_id_rel(Cls):
    return relationship(
        Cls,
        uselist=False,
        lazy=False,  # This is just my use case, not needed
        primaryjoin=lambda: and_(
            Diff.foreign_table == Cls.__tablename__,
            foreign(Diff.foreign_id) == Cls.id,
        ),
        backref=backref(
            'diffs',
            uselist=True,
            lazy=True,  # This is just my use case, not needed
            primaryjoin=lambda: and_(
                Diff.foreign_table == Cls.__tablename__,
                foreign(Diff.foreign_id) == Cls.id,
            ),
        ),
    )


class Diff(Model):
    ...

    foo = _foreign_id_rel(Foo)
    bar = _foreign_id_rel(Bar)
    baz = _foreign_id_rel(Baz)

    @property
    def foreign(self):
        if self.foreign_table:
            return getattr(self, self.foreign_table)

    @foreign.setter
    def foreign(self, value):
        if value is None:
            self.foreign_table = None
            self.foreign_id = None
        else:
            tbl_name = value.__tablename__
            self.foreign_table = tbl_name
            setattr(self, tbl_name, value)

【讨论】：