'Adds a check if the deletion was originally intended to be done
individually or from a nested deletion when an attachment and all
of its revisions was supposed to be deleted.
individual == True means only the revision should be deleted and
therefor the check if there are other sibling revisions is moot.'

def delete(self, username=None, individual=True, *args, **kwargs):

    if (individual and (self.siblings().count() == 0)):
        raise IntegrityError((u'You    cannot    delete    the    last    revision    of    attachment    %s' % self.attachment))
    trash_item = self.trash(username=username)
    super(AttachmentRevision, self).delete(*args, **kwargs)
    return trash_item
