Nested Models in Backbone.js, how to approach

I have the very same issue while I'm writing my Backbone application. Having to deal with embedded/nested models. I did some tweaks that I thought was a quite elegant solution.

Yes, you can modify the parse method to change a attributes around in the object, but all of that is actually pretty unmaintainable code IMO, and feels more of a hack than a solution.

Here's what I suggest for your example:

First define your Layout Model like so.

var layoutModel = Backbone.Model.extend({});

Then here's your image Model:

var imageModel = Backbone.Model.extend({

    model: {
        layout: layoutModel,
    },

    parse: function(response){
        for(var key in this.model)
        {
            var embeddedClass = this.model[key];
            var embeddedData = response[key];
            response[key] = new embeddedClass(embeddedData, {parse:true});
        }
        return response;
    }
});

Notice that I have not tampered with the model itself, but merely pass back the desired object from the parse method.

This should ensure the structure of the nested model when you're reading from the server. Now, you would notice that saving or setting is actually not handled here because I feel that it makes sense for you to set the nested model explicitly using the proper model.

Like so:

image.set({layout : new Layout({x: 100, y: 100})})

Also take note that you are actually invoking the parse method in your nested model by calling:

new embeddedClass(embeddedData, {parse:true});

You can define as many nested models in the model field as you need.

Of course, if you want to go as far as saving the nested model in its own table. This wouldn't be sufficient. But in the case of reading and saving the object as a whole, this solution should suffice.

I'm posting this code as an example of Peter Lyon's suggestion to redefine parse. I had the same question and this worked for me (with a Rails backend). This code is written in Coffeescript. I made a few things explicit for people unfamiliar with it.

class AppName.Collections.PostsCollection extends Backbone.Collection
  model: AppName.Models.Post

  url: '/posts'

  ...

  # parse: redefined to allow for nested models
  parse: (response) ->  # function definition
     # convert each comment attribute into a CommentsCollection
    if _.isArray response
      _.each response, (obj) ->
        obj.comments = new AppName.Collections.CommentsCollection obj.comments
    else
      response.comments = new AppName.Collections.CommentsCollection response.comments

    return response

or, in JS

parse: function(response) {
  if (_.isArray(response)) {
    return _.each(response, function(obj) {
      return obj.comments = new AppName.Collections.CommentsCollection(obj.comments);
    });
  } else {
    response.comments = new AppName.Collections.CommentsCollection(response.comments);
  }
  return response;
};

Use Backbone.AssociatedModel from Backbone-associations :

    var Layout = Backbone.AssociatedModel.extend({
        defaults : {
            x : 0,
            y : 0
        }
    });
    var Image = Backbone.AssociatedModel.extend({
        relations : [
            type: Backbone.One,
            key : 'layout',
            relatedModel : Layout          
        ],
        defaults : {
            name : '',
            layout : null
        }
    });

I'm not sure Backbone itself has a recommended way to do this. Does the Layout object have its own ID and record in the back end database? If so you can make it its own Model as you have. If not, you can just leave it as a nested document, just make sure you convert it to and from JSON properly in the save and parse methods. If you do end up taking an approach like this, I think your A example is more consistent with backbone since set will properly update attributes, but again I'm not sure what Backbone does with nested models by default. It's likely you'll need some custom code to handle this.

I'd go with Option B if you want to keep things simple.

Another good option would be to use Backbone-Relational. You'd just define something like:

var Image = Backbone.Model.extend({
    relations: [
        {
            type: Backbone.HasOne,
            key: 'layout',
            relatedModel: 'Layout'
        }
    ]
});