is_valid for ModelForm changes instance if instance is provided

[italomaia]

See this example:

def val_check(value):
  from django.core.exceptions import ValidationError
  raise ValidationError('Some reason')

class Model(models.Model):
  image = models.ImageField(upload_to='some/place', validators=[val_check(])

class ModelForm(forms.ModelForm):
  pass

def view(request, model_id):
  instance = Model.objects.get(pk=model_id)
  if request.method=='POST':
    form = ModelForm(request.POST, instance=instance)
  else: form = ModelForm(instance=instance)
  if form.is_bound and form.is_valid():
    form.save()
    # do something else
  return simple.direct_to_template(request, {'template':'some.html', 'extra_context':{'model':instance}})

In the example above, if i send a post request with some data for the field image, instance will be modified. If i use the value of instance in my 'some.html' i'll get invalid data in my template. This seems wrong.

[anonymous]

It is not clear from the report what this ticket is reporting, it doesn't help at all that the example code isn't correct/complete:

• The view isn't instanciating the ModelForm with request-FILES, something absolutely necessary when dealing with file uploads.
• The ModelForm definition is incomplete.
• The code seems to be using some third party simple.direct_to_template() function.
• The validators argument value has a syntax error.

If the OP or anyone wants to reopen this, please provide a working test case and a clear description of the issue at hand.

Remember Since Django 1.2 ModelFom validation also triggers Model validation

[Ramiro Morales]

Oops, anonymous that close the ticket was me.

[Torsten Bronger]

If necessary I will create a snippet for reproducing this, but actually the summary is complete: If you pass an "instance" to a model form constructor and later on call ".is_valid()" on that form, that given instance is changed in place. Very often you don't care about this object. But for example, if you want to compare the given instance with the new one returned by my_model_form.save(), changes are never detected.

I cannot read this behaviour from the ​docs. It is not about model validation here: The fields are really updated and not just normalised.

See also ​http://permalink.gmane.org/gmane.comp.python.django.user/132931 and ​http://groups.google.com/group/django-users/msg/2e31bfaefd8351cf.

[Aymeric Augustin]

Accepting as a documentation issue. We could make it clearer that calling is_valid() will update the instance passed to the ModelForm.

[stumbles]

Explain that is_valid() changes the model instance in place.

[stumbles]

I've added a patch to point out both that is_valid() causes the model instance to be modified in place, and also that this modification can be partial.

[Tim Graham <timograham@…>]

In [3fd89d99036696ba08dd2dd7e20a5b375f85d23b]:

Fixed #14885 - Clarified that ModelForm cleaning may not fully complete if the form is invalid.

Thanks Ben Sturmfels for the patch.

[Tim Graham <timograham@…>]

In [42aee6ffe5f16852347e0cf069447950e9d2ef85]:

[1.4.x] Fixed #14885 - Clarified that ModelForm cleaning may not fully complete if the form is invalid.

Thanks Ben Sturmfels for the patch.

Backport of 3fd89d99036696ba08dd2dd7e20a5b375f85d23b from master.