diff --git a/lib/Doctrine/ORM/EntityRepository.php b/lib/Doctrine/ORM/EntityRepository.php
index 1b0619efe..17cc29807 100644
--- a/lib/Doctrine/ORM/EntityRepository.php
+++ b/lib/Doctrine/ORM/EntityRepository.php
@@ -117,7 +117,7 @@ class EntityRepository implements ObjectRepository
             }
             $sortedId[$identifier] = $id[$identifier];
         }
-        
+
         // Check identity map first
         if ($entity = $this->_em->getUnitOfWork()->tryGetById($sortedId, $this->_class->rootEntityName)) {
             if ( ! ($entity instanceof $this->_class->name)) {
diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index f02083d0d..a026d001a 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -1647,7 +1647,20 @@ class UnitOfWork implements PropertyChangedListener
 
                 $this->persistNew($class, $managedCopy);
             } else {
-                $managedCopy = $this->tryGetById($id, $class->rootEntityName);
+                $flatId = $id;
+                if ($class->containsForeignIdentifier) {
+                    // convert foreign identifiers into scalar foreign key
+                    // values to avoid object to string conversion failures.
+                    foreach ($id as $idField => $idValue) {
+                        if (isset($class->associationMappings[$idField])) {
+                            $targetClassMetadata = $this->em->getClassMetadata($class->associationMappings[$idField]['targetEntity']);
+                            $associatedId = $this->getEntityIdentifier($idValue);
+                            $flatId[$idField] = $associatedId[$targetClassMetadata->identifier[0]];
+                        }
+                    }
+                }
+
+                $managedCopy = $this->tryGetById($flatId, $class->rootEntityName);
 
                 if ($managedCopy) {
                     // We have the entity in-memory already, just make sure its not removed.
@@ -1657,7 +1670,7 @@ class UnitOfWork implements PropertyChangedListener
                     }
                 } else {
                     // We need to fetch the managed copy in order to merge.
-                    $managedCopy = $this->em->find($class->name, $id);
+                    $managedCopy = $this->em->find($class->name, $flatId);
                 }
 
                 if ($managedCopy === null) {
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC117Test.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC117Test.php
index 9e1b5b074..c79bd2f46 100644
--- a/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC117Test.php
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC117Test.php
@@ -417,4 +417,20 @@ class DDC117Test extends \Doctrine\Tests\OrmFunctionalTestCase
 
         return $this->_em->find(get_class($editor), $editor->id);
     }
+
+    /**
+     * @group DDC-1519
+     */
+    public function testMergeForeignKeyIdentifierEntity()
+    {
+        $idCriteria = array('source' => $this->article1->id(), 'target' => $this->article2->id());
+
+        $refRep = $this->_em->find("Doctrine\Tests\Models\DDC117\DDC117Reference", $idCriteria);
+
+        $this->_em->detach($refRep);
+        $refRep = $this->_em->merge($refRep);
+
+        $this->assertEquals($this->article1->id(), $refRep->source()->id());
+        $this->assertEquals($this->article2->id(), $refRep->target()->id());
+    }
 }