/**
  * Saves an entity revision.
  *
  * @param \Drupal\Core\Entity\ContentEntityInterface $entity
  *   The entity object.
  *
  * @return int
  *   The revision id.
  */
 protected function saveRevision(ContentEntityInterface $entity)
 {
     $record = $this->mapToStorageRecord($entity->getUntranslated(), $this->revisionTable);
     $entity->preSaveRevision($this, $record);
     if ($entity->isNewRevision()) {
         $insert_id = $this->database->insert($this->revisionTable, array('return' => Database::RETURN_INSERT_ID))->fields((array) $record)->execute();
         // Even if this is a new revision, the revision ID key might have been
         // set in which case we should not override the provided revision ID.
         if (!isset($record->{$this->revisionKey})) {
             $record->{$this->revisionKey} = $insert_id;
         }
         if ($entity->isDefaultRevision()) {
             $this->database->update($this->entityType->getBaseTable())->fields(array($this->revisionKey => $record->{$this->revisionKey}))->condition($this->idKey, $record->{$this->idKey})->execute();
         }
     } else {
         $this->database->update($this->revisionTable)->fields((array) $record)->condition($this->revisionKey, $record->{$this->revisionKey})->execute();
     }
     // Make sure to update the new revision key for the entity.
     $entity->{$this->revisionKey}->value = $record->{$this->revisionKey};
     return $record->{$this->revisionKey};
 }