Return true if attribute is not changed for update_attribute

- If the attribute is not changed, then update_attribute does not run
  SQL query, this effectively means that no change was made to the
  attribute.
- This change was made in rails/rails@0fcd4cf
  to avoid a SQL call.
- But the change resulted into `nil` being returned when there was no
  change in the attribute value.
- This commit corrects the behavior to return true if there is no change
  in attribute value. This is same as previous behavior of Rails 4.2
  plus benefit of no additional SQL call.
- Fixes rails#26593.

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Return `true` from `update_attribute` when the value of the attribute
+    to be updated is unchanged.
+
+    Fixes #26593.
+
+    *Prathamesh Sonpatki*
+
 *   Serialize JSON attribute value `nil` as SQL `NULL`, not JSON `null`
 
     *Trung Duc Tran*

activerecord/lib/active_record/persistence.rb

@@ -252,7 +252,8 @@ def update_attribute(name, value)
       name = name.to_s
       verify_readonly_attribute(name)
       public_send("#{name}=", value)
-      save(validate: false) if changed?
+
+      changed? ? save(validate: false) : true
     end
 
     # Updates the attributes of the model from the passed-in hash and saves the

activerecord/test/cases/persistence_test.rb

@@ -391,14 +391,14 @@ def self.name; 'Topic'; end
     end
     topic = klass.create(title: 'Another New Topic')
     assert_queries(0) do
-      topic.update_attribute(:title, 'Another New Topic')
+      assert topic.update_attribute(:title, "Another New Topic")
     end
   end
 
   def test_update_does_not_run_sql_if_record_has_not_changed
-    topic = Topic.create(title: 'Another New Topic')
-    assert_queries(0) { topic.update(title: 'Another New Topic') }
-    assert_queries(0) { topic.update_attributes(title: 'Another New Topic') }
+    topic = Topic.create(title: "Another New Topic")
+    assert_queries(0) { assert topic.update(title: "Another New Topic") }
+    assert_queries(0) { assert topic.update_attributes(title: "Another New Topic") }
   end
 
   def test_delete