Update rollback methods, not to behave like undo

Fix behavior for rollbackAttributes. The intent is to rollback to the value
fetched from the persistence layer; not to stash an attribute’s value on every
call to `.set()`.

- Fix failing tests for previousAttributes and rollbackAttributes
- Update `attr()` `set()` method to only assign to `.previous` once

Author: pixelhandler

addon/utils/attr.js

@@ -71,9 +71,11 @@ export default function attr(type = 'any', mutable = true) {
       this.set('attributes.' + key, value);
       if (!this.get('isNew')) {
         this._attributes[key] = this._attributes[key] || {};
+        if (this._attributes[key].previous === undefined) {
+          this._attributes[key].previous = lastValue;
+        }
         this._attributes[key].changed = value;
-        this._attributes[key].previous = lastValue;
-        const service = this.get('service');
+        let service = this.get('service');
         if (service) {
           service.trigger('attributeChanged', this);
         }

tests/unit/models/resource-test.js

@@ -208,11 +208,11 @@ test('#rollbackAttributes resets attributes based on #previousAttributes', funct
 
 test('#rollbackRelationships resets relationships', function(assert) {
   let post = createPostWithRelationships.call(this);
-  let ogAuthorId = post.get('relationships.author.data.id');
+  const ogAuthorId = post.get('relationships.author.data.id');
   let relationships = post.get('relationships');
 
   post.addRelationship('author', '5');
-  assert.notEqual(relationships.author.id, ogAuthorId, 'author changed');
+  assert.notEqual(relationships.author.data.id, ogAuthorId, 'author changed');
 
   assert.equal(relationships.comments.data.length, 1, 'one comment');
   post.removeRelationships('comments', ['3']);
@@ -221,6 +221,19 @@ test('#rollbackRelationships resets relationships', function(assert) {
   let changes = post.get('changedRelationships');
   assert.equal(changes.length, 2, 'two relationships were changed');
 
+  post.addRelationship('author', '6');
+  assert.equal(relationships.author.data.id, 6, 'author changed');
+  post.addRelationships('comments', ['4', '5']);
+  assert.equal(relationships.comments.data.length, 2, 'two comments added');
+
+  changes = post.get('changedRelationships');
+  assert.equal(changes.length, 2, 'two relationships were changed');
+
+  post.removeRelationship('author');
+  assert.equal(relationships.author.id, null, 'no author');
+  changes = post.get('changedRelationships');
+  assert.equal(changes.length, 2, 'two relationships were changed');
+
   post.rollbackRelationships();
 
   changes = post.get('changedRelationships');