Question

I’m trying to save a ModelForm as I did a thousand times before, but I’m not getting it to work. This is my Test class:

class FooForm(ModelForm):
    class Meta:
        model = Account

Now I simple get an object and save it:

a = Account.objects.all()[0]
a.save()

This works. Now I want to pass this object to the form and save it:

f = FooForm(instance=a)
f.is_valid()

… gives False. Even no errors are stored:

f.errors
{}
f.non_field_errors()
[]

Saving throws an error then:

f.save()
>>> AttributeError: 'FooForm' object has no attribute 'cleaned_data'

Any ideas?

[UPDATE]

Regarding to second’s comment:

f = Form(data={}, instance=a)
f.save()
[....]
ValueError: The Account could not be changed because the data didn't validate.
f.is_valid()
>>> False
f.errors
{'user': [u'This field is required.']}
>>> a.user
<User: demo>

BTW the django documentation says, passing without data is okay, then you pass an instance:

# Create a form to edit an existing Article.
>>> a = Article.objects.get(pk=1)
>>> f = ArticleForm(instance=a)
>>> f.save()

… and could anyone please clarify the downvote? I think it’s not such a strange topic.

Answer 1

Providing an instance to a ModelForm merely provides default values for the form. The form still expects POST data. You’ve given it none.

EDIT BASED ON UPDATE:

By passing an empty dataset, you’ve essentially reset everything to empty. Again, instance merely provides the default values for the form to display when it’s first rendered. It doesn’t have any impact on what final values the form will be saved with. That’s where data comes in.

Try:

f = FooForm(data=a.__dict__, instance=a)
f.is_valid()