Just to follow up on what I ended up doing - I went with approach 1 and
created a function that would add it. I also switched from __ to _ to avoid
any surprises.
Thanks again for the suggestions!
On Wednesday, May 3, 2023 at 9:27:57 PM UTC+8 Mike Bayer wrote:
> I'd be a little concerned about the double-underscore as it modifies the
> way Python getattr() works for those names, and I dont know that we have
> tests for that specific part of it, but besides that, you can always add
> new relationships or other mapped attributes to classes in one of two ways:
>
> 1. when using declarative Base, you can assign it:
> ModelClass._field_for_rel = relationship()
> 2. you can set it in the mapper:
> ModelClass.__mapper__.add_property("_field_for_rel", relationship())
>
>
> https://docs.sqlalchemy.org/en/20/orm/mapping_api.html#sqlalchemy.orm.Mapper.add_property
>
> so you could make use of that code within some kind of function that is
> applied to the class with a certain name.
>
> if you are looking to do it fully inline within the class, you might need
> to use event hooks that receive the class and mapper so that you can apply
> things after the fact, a good event for this would be "instrument_class":
> https://docs.sqlalchemy.org/en/20/orm/events.html#sqlalchemy.orm.MapperEvents.instrument_class
>
>
> On Wed, May 3, 2023, at 4:18 AM, 'Tony Cosentini' via sqlalchemy wrote:
>
> Hi,
>
> I have a pattern in some models where there is a private (as in prefixed
> with __) relationship and then a property to handle some things that need
> to happen when the relationship is fetched or written to.
>
> Currently it's implemented like this:
> class ModelClass(Base):
> __field_for_relationship = relationship('OtherModel')
> @property
> def field_for_relationship(self):
> # Some custom logic
> return self.__field_for_relationship
>
> @field_for_relationship.setter
> def field_for_relationship(self, value: OtherModel):
> # Some custom logic.
> self.__field_for_relationship = value
>
> This works, but I'd like to DRY this up into some kind of nice one-liner
> if possible because I'm using this pattern on a handful of relationships.
> However, it seems like the relationship field itself needs to be on the
> model because of some magic that happens under the hood.
>
> Any ideas on if it's possible to combine this into some kind of reusable
> utility to DRY it up?
>
> I'm not sure how important it is, but I'm still on 1.4.x, haven't made the
> jump to 2.x yet.
>
> Thanks!
> Tony
>
>
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