Question

I’m using Django model inheritance to create two models – WorkAttachmentPicture and WorkAttachmentAudio

class WorkAttachment(models.Model):
    """ Abstract class that holds all fields that are required in each attachment """
    work            = models.ForeignKey(Work)
    added           = models.DateTimeField(default=datetime.datetime.now)
    views           = models.IntegerField(default=0)

    class Meta:
        abstract = True


class WorkAttachmentFileBased(WorkAttachment):
    """ Another base class, but for file based attachments """
    description     = models.CharField(max_length=500, blank=True)
    size            = models.IntegerField(verbose_name=_('size in bytes'))

    class Meta:
        abstract = True


class WorkAttachmentPicture(WorkAttachmentFileBased):
    """ Picture attached to work """
    image           = models.ImageField(upload_to='works/images', width_field='width', height_field='height')
    width           = models.IntegerField()
    height          = models.IntegerField()

class WorkAttachmentAudio(WorkAttachmentFileBased):
    """ Audio file attached to work """
    file            = models.FileField(upload_to='works/audio')

One work can have a multiple audio and video attachments, so I’m using a modelformset_factory to create the forms:

class ImageAttachmentForm(forms.ModelForm):
    """ Image attached to work """
    image = forms.FileField(
        label=_('File'),
        help_text=_('JPEG, GIF or PNG image.')
    )
    description = forms.CharField(
        widget=forms.Textarea(),
        label=_('File description'),
        help_text=_('Max. 500 symbols.'),
        max_length=500
    )

    class Meta:
        model = WorkAttachmentPicture
        fields = ['image', 'description']

ImageAttachmentFormSet = modelformset_factory(WorkAttachmentPicture, form=ImageAttachmentForm)


class AudioAttachmentForm(forms.Form):
    """ Audio file attached to work """
    file = forms.FileField(
        label=_('File'),
        help_text=_('MP3 file.')
    )
    description = forms.CharField(
        widget=forms.Textarea(),
        label=_('File description'),
        help_text=_('Max. 500 symbols.'),
        max_length=500
    )

    class Meta:
        model = WorkAttachmentAudio
        fields = ['file', 'description']

AudioAttachmentFormSet = modelformset_factory(WorkAttachmentAudio, form=AudioAttachmentForm)

Everything seems correct to me, but at the project startup I get the error:

metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

If I create only one formset (ImageAttachmentFormSet, for example) everything is ok. But when I’m adding another one, error appears. How do I solve this, to use modelformsets with inherited models?

Answer 1

Solved it. If you look carefully

# this has forms.ModelForm
class ImageAttachmentForm(forms.ModelForm):
# this has forms.Form
class AudioAttachmentForm(forms.Form):

I’ve changed forms.Form to forms.ModelForm and everything works now – this was a simple copy/paste error.