会话中流星存储和检索自定义对象

Question

I have a custom shopping cart object that I created and put it in the lib folder.

ShoppingCart = function ShoppingCart() {
  this.Items = new Array();
  this.grandTotal = 0.00;
}

ShoppingCart.prototype.addItem = function(Item){
  this.Items.push(Item);
  this.Items.sort();
  this.calculateTotal();
}

I initialized the shopping cart and store it as Session.set('shoppingCart') during the page created phase.

Template.loginStatus.created = function() {
  Session.set('loginShown',false);
  if(!Session.get('shoppingCart')){ //set default if session shopping cart not exist
    var cart = new ShoppingCart();
    Session.setDefault('shoppingCart',cart);
  }

Then when user click add item to cart, it will trigger this logic:

var cart = Session.get('shoppingCart');
cart.addItem(item);
Session.set('shoppingCart',cart);

Somehow, it does not work. When I take a look ad the chrome console it says undefined is not a function , pointing at cart.addItem(item) line. If I change it to this, it will work , but of course since everytime new shopping cart is created, I cannot accumulate items in the cart.

var cart = new ShoppingCart();
cart.addItem(item);
Session.set('shoppingCart',cart);

How should I store and retrieve the object from session properly? It looks like the returned object from the Session.get() somehow not considered as ShoppingCart. Did I miss any type cast?

Answer 1

As @Peppe LG mentioned, you can only store EJSONs in Session. To store your custom object, you need to be able to manually transform it to and from EJSONs. Example:

_.extend(ShoppingCart, {
  fromJSON: function(json) {
    var obj = new ShoppingCart();
    obj.grandTotal = json.grandTotal;
    obj.Items = json.Items;
    return obj;
  },
});

_.extend(ShoppingCart.prototype, {
  toJSON: function() {
    return {
      grandTotal: this.grandTotal,
      Items: this.Items,
    };
  },
});

Then you can save it to Session:

Session.set('shoppingCart', cart.toJSON());

and restore:

ShoppingCart.fromJSON(Session.get('shoppingCart'));

Answer 2

I ran into the same problem. Essentially what is happening Meteor Sessions (and Collections) can only store EJSON types, so your ShoppingCart custom type is retrieved from the Session as a normal Object.

While you can manually transform to and from EJSONs, you may end up needing to do this repeatedly in a lot of different places. If your ShoppingCart is a member of another object, you'll have to also manually transform the member. It's better to use EJSON.addType to tell Meteor how to handle it automatically anywhere you store or retrieve an object of that type.

There's a great demo of this here: https://www.eventedmind.com/feed/meteor-create-a-custom-ejson-type . Full docs are also here: http://docs.meteor.com/#/full/ejson . But a short version is this:

1. Add a method to your custom type called typeName:

   \nShoppingCart.prototoype.typeName = function(){ \n    return "ShoppingCart"; \n}; \n

2. Add another method called toJSONValue:

   \nShoppingCart.prototype.toJSONValue = function(){ \n    /* return a JSON compatible version of your object */ \n}; \n

3. And finally, add the custom type to EJSON with:

   \nEJSON.addType("ShoppingCart", function fromJSONValue(value){ \n    /* return an object of your custom type from the JSON object 'value' */ \n}; \n

NOTE: the "Type Name" in steps 1 and 3 must match exactly.