FIX: Duplicating many_many relationships looses the extra fields in 4.0

src/ORM/DataObject.php

@@ -459,8 +459,20 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         // Copy all components from source to destination
         $source = $sourceObject->getManyManyComponents($manyManyName);
         $dest = $destinationObject->getManyManyComponents($manyManyName);
+
+        if ($source instanceof UnsavedRelationList) {
+            $extraFieldNames = array();
+        } else {
+            $extraFieldNames = $source->getExtraFields();
+        }
+
         foreach ($source as $item) {
-            $dest->add($item);
+            // Merge extra fields
+            $extraFields = array();
+            foreach ($extraFieldNames as $fieldName => $fieldType) {
+                $extraFields[$fieldName] = $item->getField($fieldName);
+            }
+            $dest->add($item, $extraFields);
         }
     }
 

tests/php/ORM/DataObjectDuplicationTest.php

@@ -102,7 +102,7 @@ class DataObjectDuplicationTest extends SapphireTest
 
         //create relations
         $one->twos()->add($two);
-        $one->threes()->add($three);
+        $one->threes()->add($three, array('TestExtra'=>'three'));
 
         $one = DataObject::get_by_id(DataObjectDuplicationTest\Class1::class, $one->ID);
         $two = DataObject::get_by_id(DataObjectDuplicationTest\Class2::class, $two->ID);
@@ -161,6 +161,12 @@ class DataObjectDuplicationTest extends SapphireTest
             $threeCopy->ones()->First()->ID,
             "Match between relation of copy and the original"
         );
+
+        $this->assertEquals(
+            'three',
+            $oneCopy->threes()->byID($three->ID)->TestExtra,
+            "Match between extra field of copy and the original"
+        );
     }
 
     public function testDuplicateManyManyFiltered()

tests/php/ORM/DataObjectDuplicationTest/Class1.php

@@ -20,4 +20,10 @@ class Class1 extends DataObject implements TestOnly
     private static $many_many = array(
         'threes' => Class3::class
     );
+
+    private static $many_many_extraFields = array(
+        'threes' => array(
+            'TestExtra' => 'Varchar'
+        )
+    );
 }