django - comparing old and new field value before saving

Question

I have a django model, and I need to compare old and new values of field BEFORE saving.

I\'ve tried the save() inheritence, and pre_save signal. It was triggered cor

Answer 1

I agree with Sahil that it is better and easier to do this with ModelForm. However, you would customize the ModelForm's clean method and perform validation there. In my case, I wanted to prevent updates to a model's instance if a field on the model is set.

My code looked like this:

from django.forms import ModelForm

class ExampleForm(ModelForm):
    def clean(self):
        cleaned_data = super(ExampleForm, self).clean()
        if self.instance.field:
            raise Exception
        return cleaned_data