diff --git a/lib/Doctrine/ORM/PersistentCollection.php b/lib/Doctrine/ORM/PersistentCollection.php
index afbbf361f..6cb697c75 100644
--- a/lib/Doctrine/ORM/PersistentCollection.php
+++ b/lib/Doctrine/ORM/PersistentCollection.php
@@ -199,7 +199,7 @@ final class PersistentCollection implements Collection
      */
     private function _initialize()
     {
-        if ( ! $this->_initialized) {
+        if ( ! $this->_initialized && $this->_association) {
             if ($this->_isDirty) {
                 // Has NEW objects added through add(). Remember them.
                 $newObjects = $this->_coll->toArray();
@@ -580,7 +580,7 @@ final class PersistentCollection implements Collection
      */
     public function __sleep()
     {
-        return array('_coll');
+        return array('_coll', '_initialized');
     }
     
     /* ArrayAccess implementation */
diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index 8287d1fa7..3aefcc6c5 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -1366,6 +1366,7 @@ class UnitOfWork implements PropertyChangedListener
                 if ( ! isset($class->associationMappings[$name])) {
                     $prop->setValue($managedCopy, $prop->getValue($entity));
                 } else {
+                    // why $assoc2? See the method signature, there is $assoc already!
                     $assoc2 = $class->associationMappings[$name];
                     if ($assoc2->isOneToOne()) {
                         if ( ! $assoc2->isCascadeMerge) {
@@ -1379,6 +1380,12 @@ class UnitOfWork implements PropertyChangedListener
                             }
                         }
                     } else {
+                        $mergeCol = $prop->getValue($entity);
+                        if ($mergeCol instanceof PersistentCollection && !$mergeCol->isInitialized()) {
+                            // keep the lazy persistent collection of the managed copy.
+                            continue;
+                        }
+
                         $coll = new PersistentCollection($this->_em,
                                 $this->_em->getClassMetadata($assoc2->targetEntityName),
                                 new ArrayCollection
diff --git a/tests/Doctrine/Tests/Models/CMS/CmsUser.php b/tests/Doctrine/Tests/Models/CMS/CmsUser.php
index d7f0727c0..0b900bf60 100644
--- a/tests/Doctrine/Tests/Models/CMS/CmsUser.php
+++ b/tests/Doctrine/Tests/Models/CMS/CmsUser.php
@@ -40,7 +40,7 @@ class CmsUser
      */
     public $address;
     /**
-     * @ManyToMany(targetEntity="CmsGroup", inversedBy="users", cascade={"persist"})
+     * @ManyToMany(targetEntity="CmsGroup", inversedBy="users", cascade={"persist", "merge"})
      * @JoinTable(name="cms_users_groups",
      *      joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
      *      inverseJoinColumns={@JoinColumn(name="group_id", referencedColumnName="id")}
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC501Test.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC501Test.php
new file mode 100644
index 000000000..a40c5c7cd
--- /dev/null
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC501Test.php
@@ -0,0 +1,128 @@
+<?php
+namespace Doctrine\Tests\ORM\Functional\Ticket;
+
+use Doctrine\Tests\OrmFunctionalTestCase;
+use Doctrine\Tests\Models\CMS\CmsUser;
+use Doctrine\Tests\Models\CMS\CmsGroup;
+use Doctrine\Tests\Models\CMS\CmsPhonenumber;
+use Doctrine\Tests\Models\CMS\CmsAddress;
+use Doctrine\ORM\UnitOfWork;
+
+require_once __DIR__ . '/../../../TestInit.php';
+
+/**
+ * ----------------- !! NOTE !! --------------------
+ * To reproduce the manyToMany-Bug it's necessary
+ * to cascade "merge" on cmUser::groups
+ * -------------------------------------------------
+ *
+ * @PHP-Version 5.3.2
+ * @PHPUnit-Version 3.4.11
+ *
+ * @author markus
+ */
+class DDC501Test extends OrmFunctionalTestCase {
+    protected function setUp() {
+        $this->useModelSet('cms');
+        parent::setUp();
+    }
+
+    public function testCreateUser()
+    {
+        ### Create User
+        $user = $this->createAndPersistUser();
+        $this->_em->flush();
+
+        $this->assertTrue($this->_em->contains($user));
+        $this->_em->clear();
+        $this->assertFalse($this->_em->contains($user));
+
+        unset($user);
+
+        ### Reload User from DB *without* any associations
+        $userReloaded = $this->loadUserFromEntityManager();
+
+        $this->assertTrue($this->_em->contains($userReloaded));
+        $this->_em->clear();
+        $this->assertFalse($this->_em->contains($userReloaded));
+
+        // freeze and unfreeze
+        $userClone = unserialize(serialize($userReloaded));
+        $this->assertType('Doctrine\Tests\Models\CMS\CmsUser', $userClone);
+
+        // detached user can't know about his phonenumbers
+        $this->assertEquals(0, count($userClone->getPhonenumbers()));
+
+        // detached user can't know about his groups either
+        $this->assertEquals(0, count($userClone->getGroups()));
+
+        ### Merge back and flush
+        $userClone = $this->_em->merge($userClone);
+
+        /*
+		 * Back in managed world I would expect to have my phonenumbers back but they aren't!
+		 * Remember I didn't touch (and propably didn't need) them at all while in detached mode.
+        */
+        $this->assertEquals(4, count($userClone->getPhonenumbers()), 'Phonenumbers are not available anymore');
+
+        /*
+		 * This works fine as long as cmUser::groups doesn't cascade "merge"
+        */
+        $this->assertEquals(2, count($userClone->getGroups()));
+
+        $this->_em->flush();
+        $this->_em->clear();
+
+        $this->assertFalse($this->_em->contains($userClone));
+
+        ### Reload user from DB
+        $userFromEntityManager = $this->loadUserFromEntityManager();
+
+        /*
+     * Strange: Now the phonenumbers are back again
+        */
+        $this->assertEquals(4, count($userFromEntityManager->getPhonenumbers()));
+
+        /*
+		 * This works fine as long as cmUser::groups doesn't cascade "merge"
+		 * Otherwise group memberships are physically deleted now! 
+        */
+        $this->assertEquals(2, count($userClone->getGroups()));
+    }
+
+    protected function createAndPersistUser()
+    {
+        $user = new CmsUser();
+        $user->name = 'Luka';
+        $user->username = 'lukacho';
+        $user->status = 'developer';
+
+        foreach(array(1111,2222,3333,4444) as $number) {
+            $phone = new CmsPhonenumber;
+            $phone->phonenumber = $number;
+            $user->addPhonenumber($phone);
+        }
+
+        foreach(array('Moshers', 'Headbangers') as $groupName) {
+            $group = new CmsGroup;
+            $group->setName($groupName);
+            $user->addGroup($group);
+        }
+
+        $this->_em->persist($user);
+
+        return $user;
+    }
+
+    /**
+     * @return Doctrine\Tests\Models\CMS\CmsUser
+     */
+    protected function loadUserFromEntityManager()
+    {
+        return $this->_em
+                ->createQuery('SELECT u FROM Doctrine\Tests\Models\CMS\CmsUser u WHERE u.name like :name')
+                ->setParameter('name', 'Luka')
+                ->getSingleResult();
+    }
+
+}
\ No newline at end of file