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

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Mar 27, 2014, 1:04:23 AM3/27/14

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#22346: ModelForm modifies the Model instance before save is called
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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
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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.

--
Ticket URL: <https://code.djangoproject.com/ticket/22346>
Django <https://code.djangoproject.com/>
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Mar 27, 2014, 1:11:59 AM3/27/14

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

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.