Django: Denying model saves based on particular criteria

Question

I have a Django model, that is incredibly simple:

class Person(models.Model):
    name = models.CharField(max_length=100)

I want to deny saving of this model if the actual name changes, but I want to allow changes to capitalisation. So for example:

SAM -> sAm: allowed
Sam -> SAM: allowed
Sam -> John: not allowed

How can I override the save() method of my Person model so that such edits are denied? Particularly, I'm struggling with:

1. Gaining access the pre-save version of the object in the save() method.
2. Showing a message to the user within Django's admin area when a save is denied.
3. Returning a user back to the edit screen when a save is denied.

Feel free to answer any part of the question on its own, and thanks in advance!

Answer 1

This answer has two good methods to detect whether a field has changed and do something.

In your case you'd modify it to not just detect if a field has changed but also detect if it's a change you want to allow.

Answer 2

I would use a form and some custom validation in the "clean" method:

example:

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

    def clean(self):
        cleaned_data = self.cleaned_data
        name = cleaned_data.get("name ")
        if name == ###:
            #custom validition checking here
            raise forms.ValidationError('You can only capitalize.')
        return cleaned_data