Example.dql

SELECT RELATION_ID(p.author) AS author_id
FROM Post AS p

RelationId.php

<?php

namespace FSC\Doctrine\DQL;

use Doctrine\ORM\Query\AST\Functions\FunctionNode;
use Doctrine\ORM\Query\Lexer;
use Doctrine\ORM\Query\Parser;
use Doctrine\ORM\Query\SqlWalker;

/**
 * @author Adrien Brault <[email protected]>
 */
class RelationId extends FunctionNode
{
    private $field;

    public function parse(Parser $parser)
    {
        $parser->match(Lexer::T_IDENTIFIER);
        $parser->match(Lexer::T_OPEN_PARENTHESIS);
        $this->field = $parser->AssociationPathExpression();
        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
    }

    public function getSql(SqlWalker $sqlWalker)
    {
        return $sqlWalker->walkPathExpression($this->field);
    }
}