与SQLalchemy中不同表的关系 [英] Relationship to different tables in SQLalchemy
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.
I have multiple tables. All tables have multiple columns that could not logically be stored in one merged table.
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,我要将更改保存在历史记录表中:
Now I want to save changes in a history table:
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:当插入新的 Foo
, Bar
时,如何在同一提交中填充 Diff.foreign_id
或 Baz
?
Now my problem is: How can I populate Diff.foreign_id
in the same commit when inserting a new Foo
, Bar
, or Baz
?
这可行,但是如果插入 Diff
会出现问题,那么回滚对 foo
的更改为时已晚:
This works, but if there should be a problem to insert the Diff
then it is too late to rollback the changes to foo
:
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 :-(')
对于正常关系,将自动插入ID:
For normal relationships the id is inserted automagically:
class FooDiff(Model):
diff_id = Column(...)
foo_id = Column(Integer(unsigned=True), ForeignKey(...), ...)
foo = relationship(Foo)
但是我没有 Diff.foo
,因为 Diff.foreign_id
可以指向许多不同的表.
But I don't have Diff.foo
, because Diff.foreign_id
could point to many different tables.
推荐答案
我想出的解决方案与@van 已链接: ORM示例:通用关联.
The solution I came up with is almost the same as that what @van has linked: 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)
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