[2.0] Ensuring that PersistentCollection instances that are injected by Doctrine can be transparently serialized.

lib/Doctrine/ORM/PersistentCollection.php

@@ -117,6 +117,8 @@ final class PersistentCollection extends \Doctrine\Common\Collections\Collection
      */
     private $_isDirty = false;
 
+    private $_isSerializing = false;
+
     /**
      * Creates a new persistent collection.
      */
@@ -427,4 +429,18 @@ final class PersistentCollection extends \Doctrine\Common\Collections\Collection
     {
         $this->_isDirty = $dirty;
     }
+
+    /* Serializable implementation */
+
+    /**
+     * Called by PHP when this collection is serialized. Ensures that only the
+     * elements are properly serialized.
+     *
+     * @internal Tried to implement Serializable first but that did not work well
+     *           with circular references. This solution seems simpler and works well.
+     */
+    public function __sleep()
+    {
+        return array('_elements');
+    }
 }