Monday, August 8, 2011

Backbone.js and dependent selects

I had a fairly simple development task. Once I've done 20 different ways over the years. I have a select control with customers. When it changes I need to load the set of customer shipping addresses. The client code was a combination of jquery ajax calls and a custom client library for rendering/managing to server restful resources. Pretty quickly the code was getting messy! The problem seemed so easy, but the code did not reflect that. I decided to refactor it using backbone.js. I'm going to show you how I solved this problem in this post. It will not cover the basics of backbone.js. It will cover a simple use case and the design I came up with. It also covers some caveats I discovered with backbone.js.

Finally backbone.js has entered my development toolkit. I was waiting for it without even knowing it. It addresses so many issues with typical client code development.

If you don't know what Backbone is, then I suggest you visit backbone.js. They have a number of tutorials. (Including this one.) The docs are solid. Once you bone up on the basics come on back.

One of the first benefits was the code organization. With Backbone and Jammit (for rails) I can layout a really nice development tree with each class in its own file. Here is the tree I currently have setup:

├── app
│   ├── helpers
│   ├── models
│   │   └── collections
│   ├── templates
│   └── views
└── core
├── helpers
├── models
├── templates
│   └── controls
└── views
└── controls

I am able to have a 'core' set of helpers, models, templates, and views that are used across applications. This is not really a backbone feature, but because of backbone I setup Jammit this way.

The second thing I really loved was the clear separation between model and view. This just feels so natural having done the same thing on the server side for so many years. I'm going to assume you are familiar with the three basic backbone classes that I'm going to use: Model, Collection and View.

Ok the problem is how to have one select change and then once the change occurs, populate another select with a collection based on the first selected. In my case I have the models 'Customer' and 'Address'. A customer has many shipping addresses. So every time a new customer is selected a different set of shipping addresses need to be loaded.

Let's start by taking a look at the models (you'll see they are trivial).

App.Models.Customer = Backbone.Model.extend({});

App.Models.Address = Backbone.Model.extend({});

Ok those were trivial. I like trivial. Now we will look at the collections. These are bound to our select controls.

App.Collections.Customers = Backbone.Collection.extend({
model: App.Models.Customer,
url: '/customers'

App.Collections.Addresses = Backbone.Collection.extend({
model: App.Models.Address,
url: '/addresses'

Again trivial. We tell the collection what our resource url looks like.

Ok, now we get to the meat of the problem. The views. First I did was create a new view that can render a select based on a collection. It has a few other features I'll point out.

Core.Controls.Select = Backbone.View.extend({

initialize: function(){
_.bindAll(this, 'render','value','triggerDependents','selectControl','startingIndex','addModel');
this.selected = this.options.selected;
this.dependent_views = this.options.dependent_views;
// if the collection changes, re-renders
this.collection.bind("all", this.render);
"change": "triggerDependents"
collection: [], // The collection to render
selected: null, // The selected model
dependent_views: []// The dependent views to notify on change
render: function(){
selectControl: function() {return this.$("select");},
value: function() { return this.selectControl().val();},
// returns the starting index into the collection
startingIndex: function(){
var index = 0;
if(this.options.blank)index = index +1;
return index;
addModel: function(model,selected){
if(selected)this.selected = model;
return this;
triggerDependents: function(event) {

if(this.selectControl().selectedIndex>=this.startingIndex())this.selected =;
var _this = this;
view.trigger("."+event.type, _this);

Let's break this one done a little bit. Backbone has an initialize function that gets called after the object is created. In our initialize we do the following:

First we use the underscore function that will ensure that when our function is called from an event somewhere, the this variable is setup as we expect it. In Ruby or other languages it is impossible to call an object without its self or this setup correctly. Javascript is like C. You can do objects, but they are not really first class citizens. Once you get used to these quirks they are quite serviceable. Second we pull out a couple of class attributes that we expect to be passed into the constructor. By default the constructor puts these into the options hash, because they are really core to our class we pull them up. This is a style thing. I left some other options in the hash, because they were not a fundamental data structure to our view. We then bind to our collection, so if anything changes on the collection it will all our render method and redraw our view. Finally we render the control.

The next thing you see are two hashes: events and defaults. Events in views are jquery events and this hash is a shortcut for binding to them. We bind to the change event and call our function triggerDependents. Note: If you extend this 'class' and define an events hash it will take precedence over the base. The defaults hash is used to pre-populate the options with defaults. I've setup the collection and dependent_views as empty arrays (instead of null). I also setup select as null. Although null is not a very useful default , I think it documents what options the class will accept.

Now in our render we are simply putting into the html element the results of our select template. We are using the _.template method from the underscore library and Jammit is managing the template loading and compilation. Let's take a look at the template:

We are building the select control using the collection, selected and options passed in.

So far so good. Nice and clean.

I'm going to leave the rest of the select control as an exercise for the reader. I do want to point out the triggerDependents function. This will trigger the change event to any dependent_views passed into our constructor. It appends the id of the control so that a dependent view can listen to many controls and catch their events separately. We are using the event binding with backbone.js for this communication. This is independent from jquery events. it seems a little strange at first that they are not related, but the separation makes sense. If you have non-DOM events you want to trigger and handle, then use the model and view .bind and .trigger methods. The events hash does not route these non-dom events, so you have to bind to them in your initialize (or wherever is most appropriate.)

OK so were are we at. We have a select control that will notify any dependent views when they are changed. I think we are ready to actually use them. Let's start with the select for customer addresses.

App.Views.CustomersAddressSelect =  Core.Controls.Select.extend({   
initialize: function(){;
this.bind("order_customer_id.change", this.loadAddresses);
loadAddresses: function(control){
this.collection.fetch({data: {"parent_key[customer_id]": control.value()}});

Here you see we are binding to the order_customer_id.change event that will get fired by the customer select. In response to this event we reset our collection. (This will cause a render that will disable the control, while we fetch the new records.) Then we fetch the addresses passing the parent key that our controller expects to filter the addresses.

Note: From a security perspective you are going to want some server side check to make sure the given request is allowed to view the given records. That is clearly beyond the scope of this post!

OK now to hook it all up we have the following:

// our empty address collection
var addresses = new App.Collections.Addresses([]);
// Our dependent customer addresses select
var shipping_address_select = new App.Views.CustomersAddressSelect({el: $('#ship_to_address_select'),
id: 'order_shipping_address_id',
name: 'order[ shipping_address_id]',
collection: addresses});
// Our customers collection - populated from the server
var customers = new App.Collections.Customers(<%= raw select.collect{ |ct| {:name=>, :id=>}}.to_json %>);
// The customer select, here we pass in the customers collection and the shipping_address_select dependent view.
new Core.Controls.Select({el: $('#customer_select'),
id: 'order_customer_id',
name: 'order[customer_id]',
collection: customers,
blank: 'Select a customer',
// set the shipping_address_select as dependent view
// this will cause all events of the customer select to get
// fired to the dependent controls with the id.event

Wow that looks pretty simple. We are doing a couple of things here to note:

This is a erb file that is building up the collection of customers on the server. Since we sourced this code from the server, we could save the round trip to fetch the customers. We could have fetched the customers from the client just as easily.

We are binding to existing elements on the page by passing in
el: $(selector)

We are passing in the shipping_address_select control as a dependent_view, this hooks up our notifications on change.

Note: if you override the events hash in a derived class you will need to proxy any events that the base class was handling. In my case I ended up deriving a customer select class and wanted to listen to the change event. I lost my dependent view notifications, so in my change function I added a call to triggerDependents. I was not thrilled about this approach. I think preserving events in base classes would be a nice change for backbone.js.

I hope you enjoyed this post. I loved working with Backbone.js and plan on building a full scale view library that makes all the crud operations easy as this.

Cheers - Russell