Merge pull request #4896 from bagby/warn-object-at-undefined

Ember.warn when ManyArray.objectAt() is undefined

addon/-private/system/many-array.js

@@ -2,7 +2,7 @@
   @module ember-data
 */
 import Ember from 'ember';
-import { assert } from "ember-data/-private/debug";
+import { assert, warn } from "ember-data/-private/debug";
 import { PromiseArray } from "./promise-proxies";
 import { _objectIsAlive } from "./store/common";
 import diffArray from './diff-array';
@@ -136,7 +136,12 @@ export default Ember.Object.extend(Ember.MutableArray, Ember.Evented, {
   objectAt(index) {
     let object = this.currentState[index];
     //Ember observers such as 'firstObject', 'lastObject' might do out of bounds accesses
-    if (object === undefined) { return; }
+    if (object === undefined) {
+      warn(`ManyArray#objectAt(index) return undefined for index '${index}'. See https://github.com/emberjs/data/issues/4758`, false, {
+        id: 'ds.many-array.object-at-undefined'
+      });
+      return;
+    }
 
     return object.getRecord();
   },