#29173: Document that Model.save() doesn't refresh fields from the database
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Reporter: Xtreak | Owner: nobody
Type: Cleanup/optimization | Status: closed
Component: Documentation | Version: 2.0
Severity: Normal | Resolution: invalid
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by Carlton Gibson):
* status: new => closed
* resolution: => invalid
Comment:
The ''Custom Model Fields'' How-To discusses this usage explicitly in the
[https://docs.djangoproject.com/en/2.0/howto/custom-model-fields
/#preprocessing-values-before-saving Preprocessing values before saving]
section.
It suggests using `pre_save(model_instance, add)` for this kind of
behaviour.
It explicitly makes updating the model's attribute the user's
responsibility:
> You should also update the model’s attribute if you make any changes to
the value so that code holding references to the model will always see the
correct value.
Note `pre_save` takes the `model_instance` parameter precisely for this
purpose.
The canonical example of this usage is from `DateTimeField`, for handling
`auto_now` and `auto_now_add`:
{{{
def pre_save(self, model_instance, add):
if self.auto_now or (self.auto_now_add and add):
value = datetime.date.today()
setattr(model_instance, self.attname, value)
return value
else:
return super().pre_save(model_instance, add)
}}}
The quoted line from the docs was
[https://github.com/django/django/commit/8216abe74889cc867a4cf89e6708f37cec6b2e72
introduced in 2007] so this behaviour (and expected usage) is part of the
original design of the `Field` API. As such I'm going to close this
ticket.
--
Ticket URL: <https://code.djangoproject.com/ticket/29173#comment:3>
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