Let's say I have a collection in backbone and I want to be able to find the previous and next elements for a given element. Please assume that elements can be dynamically deleted and added.

var MyModel=Backbone.Model.extend({
 nextElement: function(){
//????
},
previousElement:function(){
//?????
}
});

var MyCollectionType=Backbone.Collection.extend({
model:MyModel;
});
var collection=new MyCollectionType

Let's say I have a collection in backbone and I want to be able to find the previous and next elements for a given element. Please assume that elements can be dynamically deleted and added.

var MyModel=Backbone.Model.extend({
 nextElement: function(){
//????
},
previousElement:function(){
//?????
}
});

var MyCollectionType=Backbone.Collection.extend({
model:MyModel;
});
var collection=new MyCollectionType

• javascript
• backbone.js

• 3 Shouldn't you be asking the collection for the information? – mu is too short Commented Apr 9, 2012 at 21:07
• It would probably better to put it on the collection. But if you are writing something small, it can go either way. – Joe Commented Apr 10, 2012 at 3:55

2 Answers 2

When a model is added to a collection, the collection property is added to the model which references the collection it is in. You can use this property in the nextElement and previousElement methods.

var MyModel = Backbone.Model.extend({
  initialize: function() {
    _.bindAll(this, 'nextElement', 'previousElement');
  },

  nextElement: function() {
    var index = this.collection.indexOf(this);
    if ((index + 1) === this.collection.length) {
      //It's the last model in the collection so return null
      return null;
    }
    return this.collection.at(index + 1);
  },

  previousElement: function() {
    var index = this.collection.indexOf(this);
    if (index === 0 ) {
      //It's the first element in the collection so return null
      return null;
    }
    return this.collection.at(index - 1);
  }
}

However, it seems that nextElement and previousElement are concerns the collection should have and not the model. Have you considered putting these functions on the collection rather than the model?

It was discussed as a backbone issue

https://github./jashkenas/backbone/issues/136

linssen

It can be something like this You can always get around this using a new model method:

getRelative: function(direction) {
    return this.collection.at(this.collection.indexOf(this) + direction);
}

So if you pass -1 it'll get the previous and 1 will get you the next. It'll return -1 if it doesn't find anything.