Backbone.js: Elegant way to check if data ready and if the dataset is empty

I'm looking for a better solution for two things:

  • How can I understand if the data is fetched and ready, I use BasicDealList.on("reset", function(){}) to understand if the data is fetched from ajax and parsed and ready to be used but it feels dirty.

  • If an empty JSON comes from fetching such as {}, it still shows BasicDealList.length as 1 while it should be 0 thus I was forced to check if the first element is empty via collection.length == 1 && jQuery.isEmptyObject(BasicDealList.toJSON()[0] which is very ugly.

Here is the code:

BasicDeal = Backbone.Model.extend();    
BasicDealCollection = Backbone.Collection.extend({
    model: BasicDeal,
    url: '/some/ajax/url/',
});
BasicDealList = new BasicDealCollection();

BasicDealList.on("reset", function(collection, response){
    isEmpty = collection.length == 1 && jQuery.isEmptyObject(BasicDealList.toJSON()[0]);
    if (isEmpty){
    // render no deal found html
    }
    else{ 
    // render list of deals
    }
}
BasicDealList.fetch();

Answers:

Answer

If you don't like listening for reset, you can pass a callback directly to .fetch():

BasicDealList.fetch({
    success: function(collection, response){
        // ...
    }
});

If, later in your app, you want to know whether you've fetched the data already, you could usually just check BasicDealList.length. If you want to avoid making repeated requests for collections that are actually empty on the server, you'll probably need to work out a custom solution, e.g. setting a flag on .fetch():

BasicDealList.fetch({
    success: function(collection, response){
        BasicDealList.fetched = true;
        // ...
    }
});

As for the empty data issue, you should be returning [] from the server instead of {}. Backbone's Collection calls this.add(models, ...) within .reset(), and .add() checks whether the models argument is an array; if it's not, it wraps it in one:

models = _.isArray(models) ? models.slice() : [models];

So passing {} will result in models set to [{}], which is not what you want. If you can't control the server, you could do the check for {} in a custom .parse() method, returning [] if it's found.

Answer

I know this question has already been answered but here is an alternative.

BasicDealCollection = Backbone.Collection.extend({
    model: BasicDeal,
    url: '/some/ajax/url/',
});

myCollection = new BasicDealCollection()
deferred = myCollection.fetch()

$.when(deferred).then(function() {
  // do stuff when we have the data.
});

The key benefit of this is that we are using the "when" function. The "when" function gives us the ability to check multiple fetch calls and do one success.

$.when(deferredOne, deferredTwo, deferredThree).then(function() {
  // do stuff when we have the data.
});

Also, if we stored the deferred object into a variable, we can do things like this. The variable will be a flag to let us know that we already loaded the data.

if (deferred.state() === "resolved") {
    // do stuff when we have the data.
}

When we call fetch() on a collection it returns a jQuery deferred object. A jQuery deferred object can be in 3 states, "pending", "rejected" or "resolved" and once we have the data, it will set the state of the deferred object to resolved.

Answer

We needed a way to tell if a RelationalModel's relation had been fetched or not. This is our solution (in Coffeescript).

initialize: (objects, options) ->
  @fetched = false
  @listenTo @, 'sync', -> @fetched = true

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