Django ModelForm Mixin to prevent overwriting of data if the form does not contain the POST keys

gistfile1.txt

from django.forms.models import model_to_dict
from django import forms


class OverwriteOnlyModelFormMixin(object):
    '''
    Delete POST keys that were not actually found in the POST dict
    to prevent accidental overwriting of fields due to missing POST data.
    '''
    def clean(self):
        cleaned_data = super(OverwriteOnlyModelFormMixin, self).clean()

        for field in cleaned_data.keys():
            if self.prefix is not None:
                post_key = '-'.join((self.prefix, field))
            else:
                post_key = field

            if post_key not in self.data:
                # value was not posted, thus it should not overwrite any data.
                del cleaned_data[field]

        # only overwrite keys that were actually submitted via POST.
        model_data = model_to_dict(self.instance)
        model_data.update(cleaned_data)
        return model_data