'Persist the Document form, and return the saved Document.'

def save(self, parent=None, *args, **kwargs):

    doc = super(DocumentForm, self).save(commit=False, *args, **kwargs)
    doc.parent = parent
    if ('parent_topic' in self.cleaned_data):
        doc.parent_topic = self.cleaned_data['parent_topic']
    doc.save()
    self.save_m2m()
    return doc
