JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://github.com/downloads/wycats/handlebars.js/handlebars.runtime-1.0.0.beta.6"></script>
<script src="http://cloud.github.com/downloads/emberjs/ember.js/ember-1.0.pre.min.js"></script>
<script src="http://cloud.github.com/downloads/emberjs/data/ember-data-latest.js"></script>

JavaScript

var json = {
    nodes: [
        { id: 1, children_ids: [2,3], parent_id: null },
        { id: 2, children_ids: [], parent_id: 1 },
        { id: 3, children_ids: [], parent_id: 1 }
    ]
};

var Node = DS.Model.extend({
    children: DS.hasMany(Node),
    parent:   DS.belongsTo(Node)
});

Node.FIXTURES = [
    { id: 1, children_ids: [2,3], parent_id: null },
    { id: 2, children_ids: [], parent_id: 1 },
    { id: 3, children_ids: [], parent_id: 1 }
];

var App = Ember.Application.create();

App.store = DS.Store.create({
    revision: 4,
    adapter: DS.FixtureAdapter.create()
});

var root = Node.find(1);

alert(root.get('children')); // this should be a DS.ManyArray instance
alert(root.get('children').length); // seems to return undefined (i.e. no children)