Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/modules/menu_link_content/src/Entity/MenuLinkContent.php

@@ -117,7 +117,9 @@ class MenuLinkContent extends ContentEntityBase implements MenuLinkContentInterf
    * {@inheritdoc}
    */
   public function getParentId() {
-    return $this->get('parent')->value;
+    // Cast the parent ID to a string, only an empty string means no parent,
+    // NULL keeps the existing parent.
+    return (string) $this->get('parent')->value;
   }
 
   /**
@@ -233,23 +235,20 @@ class MenuLinkContent extends ContentEntityBase implements MenuLinkContentInterf
    * {@inheritdoc}
    */
   public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
-    $fields['id'] = BaseFieldDefinition::create('integer')
-      ->setLabel(t('Entity ID'))
-      ->setDescription(t('The entity ID for this menu link content entity.'))
-      ->setReadOnly(TRUE)
-      ->setSetting('unsigned', TRUE);
+    /** @var \Drupal\Core\Field\BaseFieldDefinition[] $fields */
+    $fields = parent::baseFieldDefinitions($entity_type);
 
-    $fields['uuid'] = BaseFieldDefinition::create('uuid')
-      ->setLabel(t('UUID'))
-      ->setDescription(t('The content menu link UUID.'))
-      ->setReadOnly(TRUE);
+    $fields['id']->setLabel(t('Entity ID'))
+      ->setDescription(t('The entity ID for this menu link content entity.'));
 
-    $fields['bundle'] = BaseFieldDefinition::create('string')
-      ->setLabel(t('Bundle'))
+    $fields['uuid']->setDescription(t('The content menu link UUID.'));
+
+    $fields['langcode']->setDescription(t('The menu link language code.'));
+
+    $fields['bundle']
       ->setDescription(t('The content menu link bundle.'))
       ->setSetting('max_length', EntityTypeInterface::BUNDLE_MAX_LENGTH)
-      ->setSetting('is_ascii', TRUE)
-      ->setReadOnly(TRUE);
+      ->setSetting('is_ascii', TRUE);
 
     $fields['title'] = BaseFieldDefinition::create('string')
       ->setLabel(t('Menu link title'))
@@ -334,7 +333,7 @@ class MenuLinkContent extends ContentEntityBase implements MenuLinkContentInterf
         'type' => 'boolean',
         'weight' => 0,
       ))
-    ->setDisplayOptions('form', array(
+      ->setDisplayOptions('form', array(
         'settings' => array('display_label' => TRUE),
         'weight' => 0,
       ));
@@ -353,18 +352,6 @@ class MenuLinkContent extends ContentEntityBase implements MenuLinkContentInterf
         'weight' => -1,
       ));
 
-    $fields['langcode'] = BaseFieldDefinition::create('language')
-      ->setLabel(t('Language'))
-      ->setDescription(t('The menu link language code.'))
-      ->setTranslatable(TRUE)
-      ->setDisplayOptions('view', array(
-        'type' => 'hidden',
-      ))
-      ->setDisplayOptions('form', array(
-        'type' => 'language_select',
-        'weight' => 2,
-      ));
-
     $fields['parent'] = BaseFieldDefinition::create('string')
       ->setLabel(t('Parent plugin ID'))
       ->setDescription(t('The ID of the parent menu link plugin, or empty string when at the top level of the hierarchy.'));

core/modules/menu_link_content/src/Tests/LinksTest.php

@@ -188,6 +188,20 @@ class LinksTest extends WebTestBase {
     );
     $this->assertMenuLinkParents($links, $expected_hierarchy);
 
+    // Try changing the parent at the entity level.
+    $definition = $this->menuLinkManager->getDefinition($links['child-1-2']);
+    $entity = MenuLinkContent::load($definition['metadata']['entity_id']);
+    $entity->parent->value = '';
+    $entity->save();
+
+    $expected_hierarchy = array(
+      'parent' => '',
+      'child-1-1' => 'parent',
+      'child-1-2' => '',
+      'child-2' => 'parent',
+    );
+    $this->assertMenuLinkParents($links, $expected_hierarchy);
+
     // @todo Figure out what makes sense to test in terms of automatic
     //   re-parenting. https://www.drupal.org/node/2309531
   }

core/modules/menu_link_content/src/Tests/MenuLinkContentTranslationUITest.php

@@ -98,7 +98,10 @@ class MenuLinkContentTranslationUITest extends ContentTranslationUITestBase {
    * {@inheritdoc}
    */
   protected function doTestTranslationEdit() {
-    $entity = entity_load($this->entityTypeId, $this->entityId, TRUE);
+    $storage = $this->container->get('entity_type.manager')
+      ->getStorage($this->entityTypeId);
+    $storage->resetCache([$this->entityId]);
+    $entity = $storage->load($this->entityId);
     $languages = $this->container->get('language_manager')->getLanguages();
 
     foreach ($this->langcodes as $langcode) {