Re: [Django] #29173: Document that Model.save() doesn't refresh fields from the database

[Django]

#29173: Document that Model.save() doesn't refresh fields from the database
--+
 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
--+
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: 
Django 
The Web framework for perfectionists with deadlines.

-- 
You received this message because you are subscribed to the Google Groups 
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to django-updates+unsubscr...@googlegroups.com.
To post to this group, send email to django-updates@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/django-updates/067.de29a1a845c7adba728993bb44034186%40djangoproject.com.
For more options, visit https://groups.google.com/d/optout.

Re: [Django] #29173: Document that Model.save() doesn't refresh fields from the database

[Django]

#29173: Document that Model.save() doesn't refresh fields from the database
--+
 Reporter:  Xtreak|Owner:  nobody
 Type:  Cleanup/optimization  |   Status:  new
Component:  Documentation |  Version:  2.0
 Severity:  Normal|   Resolution:
 Keywords:| Triage Stage:  Accepted
Has patch:  0 |  Needs documentation:  0
  Needs tests:  0 |  Patch needs improvement:  0
Easy pickings:  0 |UI/UX:  0
--+

Comment (by Xtreak):

 Replying to [comment:1 Tim Graham]:
 > I think that's expected behavior as it would add non-trivial overhead to
 refresh fields from the database after each `save()`. There might be a
 hook that would allow you to do that for your custom field.

 I think we don't need to refresh the value from db. Since we know during
 `get_prep_value` function call the value to be inserted in the database
 and we can update the object with the correct value which is inserted at
 
[https://github.com/django/django/blob/a2e97abd8149e78071806a52282a24c27fe8236b/django/db/models/sql/compiler.py#L1217].
 I might be wrong here due to my limited knowledge of ORM.

 However I also think it's a breaking change and it will be good to make a
 note of this behavior in the documentation than to make this code change.

 Thanks for all the work.

-- 
Ticket URL: 
Django 
The Web framework for perfectionists with deadlines.

-- 
You received this message because you are subscribed to the Google Groups 
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to django-updates+unsubscr...@googlegroups.com.
To post to this group, send email to django-updates@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/django-updates/067.42c23f5313a917048d89e340526720a7%40djangoproject.com.
For more options, visit https://groups.google.com/d/optout.

Re: [Django] #29173: Document that Model.save() doesn't refresh fields from the database (was: Models with custom fields returns the given value instead of stored value)

[Django]

#29173: Document that Model.save() doesn't refresh fields from the database
--+
 Reporter:  Xtreak|Owner:  nobody
 Type:  Cleanup/optimization  |   Status:  new
Component:  Documentation |  Version:  2.0
 Severity:  Normal|   Resolution:
 Keywords:| Triage Stage:  Accepted
Has patch:  0 |  Needs documentation:  0
  Needs tests:  0 |  Patch needs improvement:  0
Easy pickings:  0 |UI/UX:  0
--+
Changes (by Tim Graham):

 * type:  Bug => Cleanup/optimization
 * stage:  Unreviewed => Accepted
 * component:  Database layer (models, ORM) => Documentation


Comment:

 I think that's expected behavior as it would add non-trivial overhead to
 refresh fields from the database after each `save()`. There might be a
 hook that would allow you to do that for your custom field.

-- 
Ticket URL: 
Django 
The Web framework for perfectionists with deadlines.

-- 
You received this message because you are subscribed to the Google Groups 
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to django-updates+unsubscr...@googlegroups.com.
To post to this group, send email to django-updates@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/django-updates/067.5ff005f132248a59252b259622f0bc92%40djangoproject.com.
For more options, visit https://groups.google.com/d/optout.