Doctrine query builder error on join - sql

Question

How do I write the following with doctrine query builder?
SELECT
registry.id ,
registry.couple_id,
registry.gift_id,
Sum(purchase.amount) as totalContribution,
gift.title,
gift.price
FROM
gift,
registry
LEFT JOIN
purchase ON purchase.registry_id = registry.id
WHERE
registry.gift_id = gift.id
AND registry.couple_id = 1
GROUP BY
registry.couple_id,
registry.gift_id
I tried:
$qb = $this->createQueryBuilder('g') //gift
->from('\BBB\GiftBundle\Entity\Registry', 'reg')
->select('g.id , g.title, g.description, g.price, g.imageMedium')
->addSelect('SUM(p.amount) as totalContribute')
->leftJoin('\BBB\GiftBundle\Entity\Purchase', 'p', 'ON','reg.id = p.registry')
->where('reg.gift = g.id')
->AndWhere('reg.couple = :coupleID')
->orderBy('reg.id','DESC')
->groupBy('reg.couple')
->groupBy('reg.gift')
->setParameter('coupleID', $coupleID);
But it gives me following error:
[Semantical Error] line 0, col 178 near 'p ON reg.id =': Error: Identification Variable BBB\GiftBundle\Entity\Purchase used in join path expression but was not defined before.

Answer 1

DQL can only join tables relating to the current entity (that being Registry in this DQL). Your Registry entity will need to have a declared OneToMany relation.
Assuming that this relation was called purchases you would join like so:
->leftJoin('reg.purchases', 'p')