fix regression where dynamically set id is not serialized in a create…

… save request

closes #3694
- uses es5 getters and setters to be able to set the internalModel id
- add assertion that model is new before setting id

packages/ember-data/lib/system/model/internal-model.js

@@ -111,7 +111,6 @@ InternalModel.prototype = {
     // lookupFactory should really return an object that creates
     // instances with the injections applied
     this.record = this.type._create({
-      id: this.id,
       store: this.store,
       container: this.container,
       _internalModel: this,
@@ -541,9 +540,8 @@ InternalModel.prototype = {
   },
 
   setId: function(id) {
+    Ember.assert('Object is a newly created model instance', this.isNew);
     this.id = id;
-    //TODO figure out whether maybe we should proxy
-    set(this.record, 'id', id);
   },
 
   didError: function(error) {

packages/ember-data/lib/system/model/model.js

@@ -295,7 +295,13 @@ var Model = Ember.Object.extend(Ember.Evented, {
     @property id
     @type {String}
   */
-  id: null,
+  get id () {
+    return this._internalModel.id;
+  },
+
+  set id (id) {
+    this._internalModel.setId(id);
+  },
 
   /**
     @property currentState

packages/ember-data/tests/integration/serializers/json-serializer-test.js

@@ -29,6 +29,36 @@ module("integration/serializer/json - JSONSerializer", {
   }
 });
 
+test("serialize doesn't include ID when includeId is false", function() {
+  run(function() {
+    post = env.store.createRecord('post', { title: 'Rails is omakase' });
+  });
+  var json = {};
+
+  json = env.serializer.serialize(post._createSnapshot(), { includeId: false });
+
+  deepEqual(json, {
+    title: "Rails is omakase",
+    comments: []
+  });
+});
+
+test("serialize includes id when includeId is true", function() {
+  run(function() {
+    post = env.store.createRecord('post', { title: 'Rails is omakase' });
+    post.set('id', 'test');
+  });
+  var json = {};
+
+  json = env.serializer.serialize(post._createSnapshot(), { includeId: true });
+
+  deepEqual(json, {
+    id: 'test',
+    title: 'Rails is omakase',
+    comments: []
+  });
+});
+
 test("serializeAttribute", function() {
   run(function() {
     post = env.store.createRecord('post', { title: "Rails is omakase" });

packages/ember-data/tests/unit/model-test.js

@@ -1116,3 +1116,25 @@ test('accessing attributes in the initializer should not throw an error', functi
 
   run(() => store.createRecord('person'));
 });
+
+test('setting the id after model creation should correctly update the id', function () {
+  expect(2);
+  var Person = DS.Model.extend({
+    name: DS.attr('string')
+  });
+
+  var env = setupStore({
+    person: Person
+  });
+  var store = env.store;
+
+  run(function () {
+    var person = store.createRecord('person');
+
+    equal(person.get('id'), null, 'initial created model id should be null');
+
+    person.set('id', 'john');
+
+    equal(person.get('id'), 'john', 'new id should be correctly set.');
+  });
+});