Doctrine load relations from an Arraycollection - php

Question

I'm looking for an elegant way to do the following:
Lets say I have an Entity with a OneToMany Relation, e.g.
class Parent
{
/**
* #ORM\OneToMany(targetEntity="Child")
*/
private $children;
public function __construct()
{
$this->children = new ArrayCollection();
}
}
class Child
{
/**
* #ORM\ManyToOne(targetEntity="Parent")
*/
private $parent;
}
Now in my logic I have to filter those parents which can't be done by queries only. So I end up with an ArrayCollection of parents, e.g:
$parents = new ArrayCollection([
$parent1,
$parent2,
$parent3
]);
Now from all those parents, I would like to join the children with one query.
How could I do this with doctrine.
I know I could just loop over the collection and call ->getChildren() on each parent.
But I possibly have hundreds of parents (which would mean hundreds of queries). Which I would like to avoid.
Before anyone says 'do a LEFT JOIN on the parents when you get them', I cannot filter the parents I need with only queries. So this is why I cannot just LEFT JOIN.
Cookies for thoughs !

Answer 1

In order to avoid the N + 1 selects problem, I would suggest the following solution which doesn’t need JOINs and uses two separate queries. This is the most efficient solution.
First, retrieve all parents:
$parents = $em->createQueryBuilder()
->select("p")
->from("YourFoobarBundle:Parent", "p")
->where(/*...*/)
->setParameter(/*...*/)
->indexBy("p.id")
->getQuery()->getResult();
Now load all children of those parents:
$children = $em->createQueryBuilder()
->select("c")
->from("YourFoobarBundle:Child", "c")
->where("IDENTITY(child.parent) IN (?1)")
->setParameter(1, array_keys($parents))
->getQuery()->getResult();
The great thing about Doctrine is that now all needed entities are stored in memory. So when you do a $parent->getChildren(), you don’t trigger a new DB query (unless the children have additional relations themselves).
NOTE: If you always need all children of the selected parents, you should mark the $children for eager loading:
/**
* #ORM\OneToMany(targetEntity="Child", fetch="EAGER")
*/
private $children;
In this case, Doctrine will always (!) fetch the needed children automatically.

Answer 2

You can make a sub query and use it inside a clause (a where in expression) of your other query. It is very much like #lxg describes, but you can do all this in one single query for increased performance (You don't have to execute the queries separately).
$qb = $entityManager->createQueryBuilder();
$sub = $qb->select('p')
->from('Application\Entity\Parent', 'p')
->where(/*...*/)
->setParameter(/*...*/)
$children = $qb->select('c')
->from('Application\Entity\Child', 'c')
->where($qb->expr()->in('c.parent', $sub->getDQL()))
->getQuery()
->getResult();