Django ModelForm with extra fields that are not in the model

Question

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 fie

Answer 1

I had a very similar problem except it looked like I did all the required thing, but I was getting this error when Django was starting:

django.core.exceptions.FieldError: Unknown field(s) (my_new_field) specified for MyModel

This was a silly mistake from me, I accidentally declared my field using a Widget class:

class MyForm(forms.ModelForm):
    my_new_field = forms.HiddenInput()

Instead of a Field class:

class MyForm(forms.ModelForm):
    my_new_field = forms.CharField(widget=forms.HiddenInput())

Not answering the question at hand here (which is answered well already), but might help others.