Django ModelForm with extra fields that are not in the model
First, you shouldn't have artist_id and artist fields. They are build from the model. If you need some artist name, add artist_name field, that is CharField.
Furthermore, you are trying to retrieve something from cleaned_data inside clean value. There might not be data you need - you should use values from self.data, where is data directly from POST.
I have done a ModelForm adding some extra fields that are not in the model. I use these fields for some calcualtions when saving the form.
The extra fields appear on the form and they are sent in the POST request when uploading the form. The problem is they are not added to the cleaned_data dictionary when I validate the form. How can I access them?