Question

I’ve got a Backbone model line that contains a collection of the model Stop.
At some point, I want to iterate through the stops in the line and get the total travel time along the line, using the Underscore function reduce.

This does not work, however. It seems that something happens with the collection at some point.
It seems to contain only one object without meaningful attributes, although I know for a fact that it has been populated with four stop-models with valid attributes.

The model:

App.models.Line = Backbone.Model.extend({
    initialize: function() {
        var stops = new App.models.Stops({
            model: App.models.Stop,
            id: this.get("id")
        });
        stops.fetch();
        this.set({
            stops: stops
        });
        this.set({unique: true});
        this.calculateTotalTime();
    },
    calculateTotalTime: function() {
        this.get("stops").each(function(num) {
            console.log("Model!");
        });
        console.log("Unique: ", this.get("unique"));
    }
});

Console printout is:

Model!
Unique:  true

There should be four “Model!”, since the number of models is four.

The strangest thing is that everything works just fine in the console:

window.line.get("stops").each(function(num) {
            console.log("Model!");
        });
Model!
Model!
Model!
Model!

The JS is compiled with Sprockets:

//= require ./init

//= require ./lib/jquery
//= require ./lib/underscore
//= require ./lib/backbone
//= require ./lib/raphael

//= require_tree ./controllers
//= require_tree ./models
//= require_tree ./views

//= require ./main

init.js:

window.App = {};
App.views = [];
App.models = [];

main.js:

$(function() {
  window.line = new App.models.Line({name: "4", id: 4});
  window.lineView = new App.views.Line({model: line});
  $("#outer").append(lineView.render().el);
});

Some other strange behaviour:

console.log(this.get("stops")) in the model yields this fairly normal object:

child
  _byCid: Object
  _byId: Object
  _onModelEvent: function () { [native code] }
  _removeReference: function () { [native code] }
  id: 4
  length: 4
  models: Array[4]
    0: Backbone.Model
    1: Backbone.Model
    2: Backbone.Model
    3: Backbone.Model
  length: 4
  __proto__: Array[0]
  __proto__: ctor

But calling console.log(this.get("stops").models), which should yield the array, returns only this, an array of a single object with no useful attributes:

[
  Backbone.Model
  _callbacks: Object
  _changed: false
  _changing: false
  _escapedAttributes: Object
  _previousAttributes: Object
  attributes: Object
    id: 4
    model: function (){ return parent.apply(this, arguments); }
    __proto__: Object
    cid: "c1"
    id: 4
  __proto__: Object
]

I suspect this is all down to some misunderstanding about the nature of this. Glad for any help provided.

Answer 1

stops.fetch() is an asynchronous process, so the code that you have written right after it will likely fire before the results of the fetch have come back from the server.

you’ll need to modify your code to run everything after the fetch comes back. the easiest way to do this is with the reset event from the stops collection:

App.models.Line = Backbone.Model.extend({
    initialize: function() {
        var stops = new App.models.Stops({
            model: App.models.Stop,
            id: this.get("id")
        });


        // store the stops, and then bind to the event
        this.set({stops: stops});
        stops.bind("reset", this.stopsLoaded, this);
        stops.fetch();

    },

    stopsLoaded: function(){
        // get the stops, now that the collection has been populated
        var stops = this.get("stops");
        this.set({unique: true});
        this.calculateTotalTime();
    },

    calculateTotalTime: function() {
        this.get("stops").each(function(num) {
            console.log("Model!");
        });
        console.log("Unique: ", this.get("unique"));
    }
});

the reason it works in your console is because by the time you type out the code to evaluate the model’s stops, the fetch call has already returned and populated the collection.

hope that helps