Allow Sortable objects to be compared by custom method

Sometimes it is necessary to customize the way objects should be compared.
This will help to fix issues such as deep nesting reference when we
compare objects that link to themselves.

This commit fixes #1726

Author: yellow1912

lib/Gedmo/Sortable/Comparable.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Gedmo\Sortable;
+
+/**
+ * This interface is not necessary but can be implemented for
+ * Entities which has custom compare methods
+ * Comparable
+ *
+ * @link https://wiki.php.net/rfc/comparable
+ * @author Raine Ng <[email protected]>
+ * @license MIT License (http://www.opensource.org/licenses/mit-license.php)
+ */
+interface Comparable
+{
+    public function compareTo($other);
+}

lib/Gedmo/Sortable/SortableListener.php

@@ -461,7 +461,13 @@ public function postFlush(EventArgs $args)
                                 $matches = $gr === null;
                             } elseif (is_object($gr) && is_object($value) && $gr !== $value) {
                                 // Special case for equal objects but different instances.
-                                $matches = $gr == $value;
+                                // If the object implements Comparable interface we can use its compareTo method
+                                // Otherwise we fallback to normal object comparison
+                                if ($gr instanceof Comparable) {
+                                    $matches = $gr->compareTo($value);
+                                } else {
+                                    $matches = $gr == $value;
+                                }
                             } else {
                                 $matches = $gr === $value;
                             }