Question

I have a status field which has 3 values: pending, activated
and rejected. If I am changing the value of status I want to
have a check that activated cannot be changed to pending. I
do not want to write stored-procs for this. Can I have the
previous value in Django before saving?

Means new and old value.

Answer 1

def clean_status(self):
    status = self.cleaned_data.get('status')
    if status == 'pending':
        if self.instance and self.instance.status == 'activated':
            raise forms.ValidationError('You cannot change activated to pending')

    return status

This method is to be added in a Form subclass. Its name is clean_FIELD_NAME.

cleaned_data contains previous values. New value is stored in self.instance.

Alternatively, validate() method can be added to a forms.Field subclass. See Django documentation.