Complex query join with doctrine - php

Question

I'm building a query to show items with user and then show highest Bid on the item.
Example:
Xbox 360 by james. - the highest bid was $55.
art table by mario. - the highest bid was $25.
Query
SELECT i, u
FROM AppBundle:Item i
LEFT JOIN i.user u
I have another table bids (one to many relationship). I'm not sure how can I include single highest bid of the item in the same query with join.
I know I can just run another query after this query, with function (relationship), but I'm avoiding to do that for optimisation reasons.
Solution
SQL
https://stackoverflow.com/a/16538294/75799 - But how is this possible in doctrine DQL?

Answer 1

You can use IN with a sub query in such cases.
I am not sure if I understood your model correctly, but I attempted to make your query with a QueryBuilder and I am sure you will manage to make it work with this example:
$qb = $this->_em->createQueryBuilder();
$sub = $qb;
$sub->select('mbi') // max bid item
->where('i.id = mbi.id')
->leftJoin('mbi.bids', 'b'))
->andWhere($qb->expr()->max('b.value'))
->getQuery();
$qb = $qb->select('i', 'u')
->where($qb->expr()->in('i', $sub->getDQL()))
->leftJoin('i.user', 'u');
$query = $qb->getQuery();
return $query->getResult();

Answer 2

Your SQL query may look something like
select i,u
from i
inner join bids u on i.id = u.item_id
WHERE
i.value = (select max(value) from bids where item_id = i.id)
group by i
DQL, I don't think supports subqueries, so you could try using a Having clause or see if Doctrine\ORM\Query\Expr offers anything.
To solve this for my own case, I added a method to the origin entity (item) to find the max entity in a list of entities (bids), using Doctrine's Collections' Criteria I've written about it here.
Your Item entity would contain
public function getMaxBid()
{
$criteria = Criteria::create();
$criteria->orderBy(['bid.value' => Criteria::ASC]);
$criteria->setLimit(1);
return $this->bids->matching($criteria);
}

Answer 3

Unfortunately, there's no way that i know to find the maximum bid and the bidder with one grouping query, but there's several techniques to making the logic work with several queries. You could do a sub select and that might work fine depending on the size of the table. If you're planning on growing to the point where that's not going to work, you're probably already looking at sharing your relational databases, moving some data to a less transactional, higher performance db technology, or denormalizing, but if you want to keep this implemented in pure MySQL, you could use a procedure to express in multiple commands how to check for a bid and optionally add to the list, also updating the current high bidder in a denormalized high bids table. This keeps the complex logic of how to verify the bid in one, the most rigorously managed place - the database. Just make sure you use transactions properly to stop 2 bids from being recorded concurrently ( eg, SELECT FOR UPDATE).
I used to ask prospective programmers to write this query to see how experienced with MySQL they were, many thought just a max grouping was sufficient, and a few left the interview still convinced that it would work fine and i was wrong. So good question!