Backbone / Underscore chain method with where method

backbone.js, lodash, underscore.js

Solution

The merging of some Underscore methods into collections is a little imperfect. When you say `collection.some_mixed_in_underscore_method()`, the collection unwraps some of the Backbone stuff behind your back so that the Underscore method is applied to the attributes inside the collection's models; it sort of works like this:

var ary = _(this.models).map(function(m) { return m.attributes });
return _(ary).some_mixed_in_underscore_method();

But `collection.chain()` doesn't work like that, `chain` just wraps the collection's `models` directly so if you do this:

console.log(collection.chain());

you'll see that `chain` is giving you an object that wraps an array of models. Your models won't have an `is_checked` property (i.e. there is no `model.is_checked`), they will have `is_checked` attributes though (i.e. there will be `model.get('is_checked')` and `model.attributes.is_checked`).

Now we can see where everything goes wrong:

collection.chain().where({'is_checked':true})

The models don't have `is_checked` properties. In particular, there won't be any models where `is_checked` is `true` and everything after the `where` is working with an empty array.

Now that we know where things go sideways, how do we fix it? Well, you could use `filter` instead of `where` so that you can easily unpack the models:

collection.chain()
          .filter(function(m) { return m.get('is_checked') })
          .pluck('id')
          .value();

But, your models don't have `id`s yet as you didn't create them with `id`s and you haven't talked to a server to get `id`s so you're going to get an array of `undefined`s back. If you add some `id`s:

var collection = new App.OptionCollection([
    {id: 1, 'is_checked': true},
    {id: 2, 'is_checked': true},
    {id: 3, 'is_checked': false}
]);

then you'll get the `[1,2]` that you're looking for.

Demo: http://jsfiddle.net/ambiguous/kRmaD/

Problem

There has to be something simple I am missing here. http://jsfiddle.net/v9mdZ/ I am just learning Backbone.js and Underscore.js/Lodash and am trying to get familiar with `chain`. I have the following code, which works as expected: ``` var ids = _.pluck(collection.where({'is_checked':true}), 'id'); ``` I attempted to refactor this, using `chain` like so: ``` var ids = collection.chain().where({'is_checked':true}).pluck('id').value(); ``` Why doesn't the refactored code work? Am I using `chain` wrong? Solution (details below) Don't use `where` with `chain`.

Original source