Re: [Django] #22346: ModelForm modifies the Model instance before save is called

[Django]

#22346: ModelForm modifies the Model instance before save is called
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 Reporter:  lehins@…   |Owner:  nobody
 Type:  Uncategorized  |   Status:  closed
Component:  Forms  |  Version:  1.6
 Severity:  Normal |   Resolution:  invalid
 Keywords: | Triage Stage:  Unreviewed
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 lehins@…):

 * status:  new => closed
 * needs_better_patch:   => 0
 * resolution:   => invalid
 * needs_tests:   => 0
 * needs_docs:   => 0


Comment:

 Never mind, I just found a Warning in Documentation, and it actually makes
 sense, since model fields need to be validate as well.

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

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[Django] #22346: ModelForm modifies the Model instance before save is called

[Django]

#22346: ModelForm modifies the Model instance before save is called
---+
 Reporter:  lehins@…   |  Owner:  nobody
 Type:  Uncategorized  | Status:  new
Component:  Forms  |Version:  1.6
 Severity:  Normal |   Keywords:
 Triage Stage:  Unreviewed |  Has patch:  0
Easy pickings:  0  |  UI/UX:  0
---+
 So here is a use case that is relevant to me:
 I need a model instance's primary key(pk) before I save the model, but
 only after the model form is validated. In particular I needed it to use
 in a pre_save method on a file field. So my logical solution was:

 {{{
 try:
instance = CustomModel.objects.get(pk=pk)
 except CustomModel.DoesNotExist:
instance = CustomModel()
 form = CustomModelForm(instance=instance, data=request.POST,
 files=request.FILES))
 if form.is_valid():
if not instance.pk:
instance.save() # saves the model while generating a pk
instance = form.save()
 }}}

 To my disappointment, after a few wasted hours, I figured out that binding
 data to a model instance modifies that instance, so when I am calling
 instance.save() it already contains data from the form, before even
 calling form.save() method.
 After finding this ticket: #14885 I also noticed that it may be a partial
 update of the model instance. Despite closed ticket I couldn't find it in
 current documentation.
 In my opinion it is completely illogical. Model instance should only be
 modified only upon form.save() call, this would also give a real use case
 for the existence of commit=False argument: "apply cleaned form data to
 model instance, but do not commit to database."
 Even [https://docs.djangoproject.com/en/1.6/topics/forms/modelforms/#the-
 save-method documentation] follows the logic I am proposing: "A subclass
 of ModelForm can accept an existing model instance as the keyword argument
 instance; if this is supplied, save() will update that instance"

 Overall it sounds to me like a side effect or a bug. If it truly is an
 unavoidable side-effect, I believe, we should get at least a really good
 explanation of it in the documentation.

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

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