Add an "ordered" option to hasMany attributes.

packages/ember-model/lib/has_many.js

@@ -46,6 +46,7 @@ Ember.Model.reopen({
       modelClass: type,
       content: this._getHasManyContent(key, type, embedded),
       embedded: embedded,
+      ordered: !!meta.options.ordered,
       key: key,
       relationshipKey: meta.relationshipKey,
       container: container

packages/ember-model/lib/has_many_array.js

@@ -17,22 +17,23 @@ Ember.ManyArray = Ember.RecordArray.extend({
     var originalContent = get(this, 'originalContent'),
         originalContentLength = get(originalContent, 'length'),
         content = get(this, 'content'),
-        contentLength = get(content, 'length');
+        contentLength = get(content, 'length'),
+        i, l;
 
     if (originalContentLength !== contentLength) { return true; }
 
     if (this._modifiedRecords && this._modifiedRecords.length) { return true; }
 
-    var isDirty = false;
-
-    for (var i = 0, l = contentLength; i < l; i++) {
-      if (!originalContent.contains(content[i])) {
-        isDirty = true;
-        break;
-      }
+    if (get(this, 'ordered')) {
+        for (i = 0, l = contentLength; i < l; i++) {
+            if (originalContent.objectAt(i) !== content.objectAt(i)) { return true; }
+        }
+    } else {
+        for (i = 0, l = contentLength; i < l; i++) {
+            if (!originalContent.contains(content[i])) { return true; }
+        }
     }
-
-    return isDirty;
+    return false;
   }.property('content.[]', 'originalContent.[]', '_modifiedRecords.[]'),
 
   objectAtContent: function(idx) {

packages/ember-model/tests/has_many/manipulation_test.js

@@ -283,4 +283,60 @@ test("setting a hasMany array with setObjects", function() {
 
   equal(article.get('comments.length'), 3, "should be 3 comments after revert");
   equal(article.get('comments.isDirty'), false, "should not be dirty after revert");
-});
+});
+
+test("reordering a hasMany array", function () {
+  var json = {
+    id: 1,
+    title: 'foo',
+    comments: [1, 2, 3]
+  };
+
+  var Comment = Ember.Model.extend({
+    text: attr()
+  });
+
+  var Article = Ember.Model.extend({
+    title: attr(),
+    comments: Ember.hasMany(Comment, { key: 'comments', ordered: true })
+  });
+
+  Comment.adapter = Ember.FixtureAdapter.create();
+  Comment.FIXTURES = [
+    {id: 1, text: 'uno'},
+    {id: 2, text: 'dos'},
+    {id: 3, text: 'tres'}
+  ];
+
+  var article = Article.create();
+  Ember.run(article, article.load, json.id, json);
+  // New by default for some reason
+  article.get('comments').forEach(function (comment) { comment.set('isNew', false); });
+
+  // Control test
+  article.get('comments').setObjects([Comment.find(2), Comment.find(1), Comment.find(3)]);
+  equal(article.get('comments.length'), 3, "should be 3 comments after set");
+  equal(article.get('comments.isDirty'), true, "comments should be dirty after set");
+
+  // Reorder using replace (through insertAt)
+  article.get('comments').removeAt(0);
+  article.get('comments').insertAt(1, Comment.find(2));
+  equal(article.get('comments.length'), 3, "should be 3 comments after set");
+  equal(article.get('comments.isDirty'), false, "should not be dirty after replacing to right order");
+
+  // Reorder using setObjects
+  article.get('comments').setObjects([Comment.find(2), Comment.find(1), Comment.find(3)]);
+  equal(article.get('comments.isDirty'), true, "comments should be dirty after set");
+
+  article.get('comments').setObjects([Comment.find(1), Comment.find(2), Comment.find(3)]);
+  equal(article.get('comments.length'), 3, "should be 3 comments after set");
+  equal(article.get('comments.isDirty'), false, "should not be dirty after setObjects in right order");
+
+  // Reorder using item array
+  article.get('comments').setObjects([Comment.find(2), Comment.find(1), Comment.find(3)]);
+  equal(article.get('comments.isDirty'), true, "comments should be dirty after set");
+
+  article.set('comments', [Comment.find(1), Comment.find(2), Comment.find(3)]);
+  equal(article.get('comments.length'), 3, "should be 3 comments after set");
+  equal(article.get('comments.isDirty'), false, "should not be dirty after set in right order");
+});