TIL that Doctrine 2 doesn't support LIMITs within subqueries which can be frustrating. In my case, I wanted to LEFT JOIN on a table using a subquery with a single result - something like this:

$dqb->from('MyAppBundle:Foo', 'foo')
    ->leftJoin('foo.bar', 'bar', 'WITH', 'bar = (SELECT b FROM MyAppBundle:Bar b WHERE b.foo = foo AND b.published_date >= :now ORDER BY t.startDate LIMIT 1)');

But Doctrine kept throwing this error:

Expected Doctrine\ORM\Query\Lexer::T_CLOSE_PARENTHESIS, got 'LIMIT'

To get around this limitation, I ended creating a custom Doctrine function FIRST(). Basically, you wrap a subquery within this function and Doctrine will apply LIMIT 1 for you:

$dqb->from('MyAppBundle:Foo', 'foo')
    ->leftJoin('foo.bar', 'bar', 'WITH', 'bar = FIRST(SELECT b FROM MyAppBundle:Bar b WHERE b.foo = foo AND b.published_date >= :now ORDER BY t.startDate)');

Here's what the EBNF looks like:

FirstFunction ::= "FIRST" "(" Subselect ")"

And here's the class I implemented:

<?php

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

/**
 * FirstFunction ::=
 *     "FIRST" "(" Subselect ")"
 */
class FirstFunction extends FunctionNode
{
    /**
     * @var Subselect
     */
    private $subselect;

    /**
     * {@inheritdoc}
     */
    public function parse(Parser $parser)
    {
        $parser->match(Lexer::T_IDENTIFIER);
        $parser->match(Lexer::T_OPEN_PARENTHESIS);
        $this->subselect = $parser->Subselect();
        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
    }

    /**
     * {@inheritdoc}
     */
    public function getSql(SqlWalker $sqlWalker)
    {
        return '(' . $this->subselect->dispatch($sqlWalker) . ' LIMIT 1)';
    }
}

If you're using Symfony, you can tell Doctrine about this new function by setting the following in your app/config/config.yml file:

doctrine:
    # ...
    orm:
        # ...
        dql:
            string_functions:
                FIRST: AppBundle\DBAL\FirstFunction # Your namespace might be different

It should be possible to extend this to make the LIMIT configurable - that exercise is left to the reader.