Document that refresh_from_db() doesn't clear @cached_properties

[Jarek Glowacki]

Should cached_property-decorated stuff get wiped when we call .refresh_from_db()?

I found ​this related article on the matter.

From a usage perspective, it seems like we wouldn't ever want cached properties to persist through a refresh_from_db() call, as ideally we'd like to think mymodel.refresh_from_db() is shorthand synonymous to mymodel = MyModel.objects.get(pk=mymodel.pk).

However, from an implementation perspective, maybe this would result in a rabbit hole that tries to pull through other stuff, like #26514.

Basically this ticket picks at:

Note that only fields of the model are reloaded from the database. Other database dependent values such as annotations are not reloaded.`

I have a feeling this might be a conscious design decision, but in case it isn't, thoughts?

[Tim Graham]

refresh_from_db() doesn't know if a @cached_property is database-dependent or not. I suppose we can document this: ​PR.

[GitHub <noreply@…>]

In 95560054:

Fixed #27048 -- Documented that refresh_from_db() doesn't reload @cached_properties.

[Tim Graham <timograham@…>]

In eea74ed:

[1.10.x] Fixed #27048 -- Documented that refresh_from_db() doesn't reload @cached_properties.

Backport of 9556005425ca3d1c8c2a0e8b48e228e63a0bdd32 from master