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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