Question

I created a form class based on a model:

class MyModel(models.Model):
    increasing_field = models.PositiveIntegerField()

class MyForm(forms.ModelForm):
    class Meta:
        model = MyModel

I created a form to change an existing MyClass instance using POST
data to populate the form:

m = MyModel.objects.get(pk=n)
f = MyForm(request.POST, instance=m)

Every time f is being updated, f.increasing_field can only be greater
than the previous value.

How do I enforce that validation?

1 way I can think of is to have clean_increasing_field take on an extra
argument that represents the previous value of increasing_field:

def clean_increasing_field(self, previous_value)
    ...

This way I can just make sure the new value is greater than the
previous value. But it looks like clean_() methods cannot
take on extra arguments.

Any ideas on how to carry out this validation?

Answer 1

Since the original model has not been updated by the time the validation is done, you could simply look at the current (unchanged) value using “self.instance.increasing_value” (or whatever your field is called). Compare this to the new value being validated, and raise an error if it’s not higher than the current value.

def clean_increasing_field(self):
    new_val = self.cleaned_data['increasing_field']
    if new_val <= self.instance.increasing_field:
        raise forms.ValidationError("Increasing Field must increase!")
    return new_val   #must always return the data

Note: self.instance will return the underlying Model to which the ModelForm is bound.