添加一个字段并更新odoo pos订单行Odoo 11中的值

Question

我试图在pos订单行中添加一个新字段，并在按钮单击中更新值。

pos.oerder.line.py

class pos_order_line(models.Model):
    _inherit = 'pos.order.line'
    is_promo = fields.Boolean(string='Promo',default=False)

*。js

models.Orderline = models.Orderline.extend({

initialize: function() {
  _super_orderline.initialize.apply(this,arguments);
  this.is_promo = false;
},

// Help me to understand this function
export_as_JSON: function(){ 
    var json = _super_orderline.export_as_JSON.apply(this,arguments);
    json.is_promo = this.is_promo;
    return json;
},

// Help me to understand this function
init_from_JSON: function(json){
    _super_orderline.init_from_JSON.apply(this,arguments);
    this.is_promo = json.is_promo;
},

});

var OfferButton = screens.ActionButtonWidget.extend({
'template': 'PromotioanlButton',
button_click: function(){
    var self = this;
    self.get_promo_config();
},


get_promo_config: function () {
  var self = this;
  var order    = this.pos.get_order();
  var orderline = order.get_orderlines();
  var order_product_ids = []
  _.each(orderline,function(line){
    order_product_ids.push({'product_id':line.product.id,'qty':line.quantity});
  });

  rpc.query({
    model:'promo.config',
    method:'check_promo_offer',
    args :[order_product_ids],
  }).then(function(product_ids){

    _.each(product_ids, function(pid){
      var product  = self.pos.db.get_product_by_id(pid.product_id);


 **// I added new item into cart using below code and its working fine. 
 //It add all the fields below mention except `is_promo`, i mean it still false. 
  //how can i make it true.**
      order.add_product(product, { price:0.0 ,quantity:pid.product_uom_qty,discount:25,is_promo:true,merge:false});
  });
  });
},
});