Hi there,
I'm trying to create a relationship for a Mxin that is agnostic to what the
primary key of the mixed object is.
Basically we have this:
class TransitionBase(SurrogatePK, Model):
__abstract__ = True
obj_id = Column(String, nullable=False, index=True)
state = Column(String, nullable=False, index=True)
...(more stuff)...
class HasStateMixin:
@staticmethod
def get_state_class() -> Type[TransitionBase]:
raise NotImplementedError()
@declared_attr
def transitions(cls) -> List[TransitionBase]:
state_cls = cls.get_state_class()
return relationship(
state_cls,
primaryjoin=foreign(state_cls.obj_id) == remote(cls.id), #
type: ignore
order_by=state_cls.id.desc(),
)
And it works well, as long as the `id` property is a normal Column. Eg:
class Transition(TransitionBase):
__tablename__ = ''transitions'
class Obj(Model, HasStateMachineMixin):
__tablename__ = 'obj'
__table_args__ = {'schema': test_schema}
id = Column(String, primary_key=True)
@staticmethod
def get_state_class() -> Type[TransitionBase]:
return Transition
But now I have a case where the primary key of the mixed class is three
columns. My first instinct was to use a hybrid property like this:
class ThreePrimaryKeys(HasStateMachineMixin):
a = Column(String, primary_key=True)
b = Column(String, primary_key=True)
c = Column(String, primary_key=True)
@hybrid_property
def id(self):
return f'/{self.a}/{self.b}/{self.c}'
@id.expression
def id(cls):
return func.concat(
'/', cls.a, '/', cls.b, '/', cls.c,
)
But that doesn't work: when I create a ThreePrimaryKeys() and then call
obj.transitions.append(MyTransition(state='foo')) it doesn't properly
persist the full concatenated ID - weirdly, it just uses column 'c' (or the
equivalent - type names have been changed to protect the innocent)
My question is: should this work? And if so, what am I doing wrong? If not,
do you have an alternate approach?
Thanks in advance,
Bobby
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