Doctrine\ODM\MongoDB\Mapping\ClassMetadataFactory::validateIdentifier PHP Method

    protected function validateIdentifier($class)
    {
        if (!$class->identifier && !$class->isMappedSuperclass && !$class->isEmbeddedDocument && !$class->isQueryResultDocument) {
            throw MappingException::identifierRequired($class->name);
        }
    }