[WIP] Partially loaded recrods

packages/ember-model/lib/attr.js

@@ -70,6 +70,10 @@ Ember.attr = function(type) {
         dataValue = data && get(data, dataKey),
         beingCreated = meta(this).proto === this;
 
+    if(this.get('isPartiallyLoaded') && this.isAttributeMissing(key)) {
+     this.missingAttributeAccessed(key);
+    }
+
     if (arguments.length === 2) {
       if (beingCreated && !data) {
         data = {};

packages/ember-model/lib/model.js

@@ -86,15 +86,32 @@ Ember.Model = Ember.Object.extend(Ember.Evented, Ember.DeferredMixin, {
   },
 
   load: function(id, hash) {
-    var data = {};
+    var data = {}, isPartiallyLoaded, attributes;
     data[get(this.constructor, 'primaryKey')] = id;
-    set(this, 'data', Ember.merge(data, hash));
+    data = Ember.merge(data, hash);
+    this._loadedAttributes = Ember.A(Ember.keys(data).map(function(key) { return this.dataKey(key); }, this));
+
+    attributes = get(this, 'attributes');
+    if(!attributes || attributes.length === 0) {
+      // TODO: fix this when implementing support for relationships
+      isPartiallyLoaded = false;
+    } else {
+      isPartiallyLoaded = Ember.EnumerableUtils.intersection(this._loadedAttributes, attributes).length !== attributes.length;
+    }
+    set(this, 'isPartiallyLoaded', isPartiallyLoaded);
+    set(this, 'data', data);
     set(this, 'isLoaded', true);
     set(this, 'isNew', false);
     this.trigger('didLoad');
     this.resolve(this);
   },
 
+  isAttributeMissing: function(key) {
+    return this.get('isPartiallyLoaded') && this._loadedAttributes && !this._loadedAttributes.contains(key);
+  },
+
+  missingAttributeAccessed: Ember.K,
+
   didDefineProperty: function(proto, key, value) {
     if (value instanceof Ember.Descriptor) {
       var meta = value.meta();

packages/ember-model/tests/partially_loaded_model_test.js

@@ -0,0 +1,50 @@
+var Model;
+
+module("Partially loaded records", {
+  setup: function() {
+    Model = Ember.Model.extend({
+      id: Ember.attr(),
+      name: Ember.attr()
+    });
+
+    Model.adapter = Ember.FixtureAdapter.create();
+  }
+});
+
+test("missingAttributeAccessed is called if the attribute is not loaded", function() {
+  expect(1);
+
+  var key, record = Model.create({ isLoaded: false });
+  Ember.run(function() {
+    record.load(1, {});
+  });
+
+  record.missingAttributeAccessed = function(_key) {
+    key = _key;
+  };
+
+  record.get('name');
+
+  equal(key, 'name', 'missingAttributeAccessed should be called with attribute name "key".');
+});
+
+test("missingAttributeAccessed is not called if the attribute is loaded", function() {
+  expect(0);
+
+  var key, record = Model.create({ isLoaded: false });
+  Ember.run(function() {
+    record.load(1, { name: 'Erik' });
+  });
+
+  record.missingAttributeAccessed = function(_key) {
+    key = _key;
+  };
+
+  record.get('name');
+
+  record.missingAttributeAccessed = function(_key) {
+    equal(true, false, "missingAttributeAccessed should not be called");
+  };
+
+  record.get('name');
+});