public function getSingleIdentifierFieldName()
    {
        if ($this->isIdentifierComposite()) {
            throw MappingException::singleIdNotAllowedOnCompositePrimaryKey($this->className);
        }

        if (! isset($this->identifier[0])) {
            throw MappingException::noIdDefined($this->className);
        }

        return $this->identifier[0];
    }