'Override unlink to delete messages and followers. This cannot be
cascaded, because link is done through (res_model, res_id).'

@api.multi
def unlink(self):

    self.env['mail.message'].search([('model', '=', self._name), ('res_id', 'in', self.ids)]).unlink()
    res = super(MailThread, self).unlink()
    self.env['mail.followers'].sudo().search([('res_model', '=', self._name), ('res_id', 'in', self.ids)]).unlink()
    return res
