Django ModelForm with extra fields that are not in the model

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.

First add the field in the form

class CreateForm(forms.ModelForm):

extra_field     = forms.CharField(label = 'extra_field', required = False)

Now while cleaning you data, you need to retrive the extra field from self.data NOT self.cleaned_data

Correct:

 self.data.get('extra_field')

Wrong:

 self.cleaned_data.get('extra_field')

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.

Ok I have resolved it. It seems that accessing the extra fields with cleaned_data['field_name'] raises a KeyError but using cleaned_data.get('field_name') works. That's weird because normal fields for the model can be accessed via cleaned_data['field_name'].

Update: No, it doesn't work. With get() it doesn't raise a KeyError but it sets a value of None because the extra fields are not in the cleaned_data dictionary.

Here is the code. In the templates there is an autocomplete, so in the form there is an "artist" field rendered as a CharField and an hidden IntegerField that will be autopopulated with the given artist id. In the clean_artist method I want to select the artist object and store it in the artist field of the form.

models.py

class Music(models.Model):
    artist = models.ForeignKey(Artist, related_name='music', blank=True, null=True)
    # other fields...

forms.py

class MusicForm(forms.ModelForm):
    artist_id = forms.IntegerField(label="", widget=forms.HiddenInput(), required=False)
    artist = forms.CharField(required=False)
    # other fields ...

    class Meta:
        model = Music

    def clean_artist(self):
        if self.cleaned_data.get('artist') == '':
            artist = None
        else:
            artist_id = self.cleaned_data.get('artist_id') # this returns always None because artist_id is not in cleaned_fields (this seemed to work with previous django versions but not with current SVN version)
            if artist_id != None:
                artist = Artist.objects.get(id=artist_id)
            else:
                artist = None

    return artist

In Django 2 you can just add the fields as it was a normal form

class CreateCompanyForm(forms.ModelForm):

    password_confirmation = forms.CharField(
        label=translate('Password confirmation'),
        max_length=70,
        widget=forms.PasswordInput(),
        required=True,
    )
    company_name = forms.CharField(
        label="Nombre de la Compañía",
        max_length=90,
        widget=forms.TextInput(),
        required=True,
    )

    class Meta:
        model = AppUser
        fields = (
            "email",
            "first_name",
            "last_name",
            "password",
        )