@api.multi
    def send_email(self, object, user_ids):
        mail_obj = object
        body_html = ''
        current_url = ''
        you = ''
        base_url = str(self.env['ir.config_parameter'].get_param('web.base.url'))  + '/web?db=' + str(http.request.session.db) + '#id=' + str(object.id) + '&view_type=form&model=modul.control.alerts&action=' +  str(self.env['ir.model.data'].get_object_reference('mail', 'email_compose_message_wizard_form')[1])
        for line in user_ids:
            user = self.env['res.users'].search([('id','=', line)])
            if user.email:
                you = you + ',' + user.email
            else :
                raise UserError(_('Please check user email, there is no user email.'))
        if you:
            outgoing_email = self.env['ir.mail_server'].search([])
            if not outgoing_email:
                raise exceptions.ValidationError(_('There is no configuration for outgoing mail server. Please contact system administrator.'))
            else:
                outgoing_email = self.env['ir.mail_server'].search([('id', '=', outgoing_email[0].id)])
                email_ids = []
                vals = {
                        'state': 'outgoing',
                        'subject': '',
                        'body_html': +'<a href="' + base_url + '">link:</a>',
                        'email_to': user.email,
                        'email_from': outgoing_email.smtp_user,
                        }
                email_ids.append(self.env['mail.mail'].create(vals))

bold code is not working, how to fix any idea ?