Django: Get previous value in clean() method

I have a model CustomModel with an IntegerField.

class CustomModel(models.Model):
    count = models.IntegerField()

When I create a new instance of CustomModel in the admin, I have to do validation, so I use the clean method and have access to the value with.

def clean(self):
    value = self.count
    ...

My problem:

When I change the instance of CustomModel, I only have access to the new, changed value but not to the original value. However, for my validation I have to compare the new value and the value before the instance got edited.

I could not found a solution how to get access. Does somebody know?

Answer

Why not take advantage of a ModelForm? Form data is saved in two steps:

1. To the form instance
2. To the model instance

So when you have a form:

class YourForm(forms.ModelForm):

    class Meta:
        model = CustomModel
        fields = ['count']

    def clean(self):
        cleaned_data = super().clean()
        count = cleaned_data.get('count')

        if count < self.instance.count:
            self.add_error('count', 'You cannot decrease the counter')

        return cleanded_data

You can then override the form within the django admin site.