/**
  * @param array $mapping
  *
  * @throws MappingException
  */
 private function _validateFieldMapping(array &$mapping)
 {
     if (!isset($mapping['fieldName'])) {
         throw MappingException::missingFieldName($this->name);
     }
     if (isset($this->fieldMappings[$mapping['fieldName']])) {
         throw MappingException::duplicateFieldMapping($this->name, $mapping['fieldName']);
     }
 }