List entities belonging to an association in Doctrine - symfony

Question

Let's say I have a "Person" entity. A person can belong to a "Group". They are associated through a ManyToMany, Join Table strategy.
The general code looks like this:
/**
* Vendor\AcmeBundle\Entity\Person
*
* #ORM\Entity(repositoryClass="Vendor\AcmeBundle\Entity\PersonRepository")
*/
class Person extends BaseUser
{
/**
* #ORM\Column(type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #ORM\ManyToMany(targetEntity="Vendor\AcmeBundle\Entity\Group")
*/
protected $groups;
}
and the group entity
/**
* #ORM\Entity
*/
class Group extends BaseGroup
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #ORM\Column(type="string", nullable=true)
*/
protected $publicName;
}
What do I want to achieve?
Given a group, list users belonging to that group in a consistent way, including pagination options (aka limit and offset)
Something like this:
function getUserFromGroup(Group $group, $criteria, $limit, $offset){};
Considerations:
The entities are mutable, they can be adapted to achieve this requisite (e.g. the association could be changed from unidirectional to bidirectional)
The amount of person entities is in the thousands (2000~8000)
The amount of groups is less than 10

Answer 1

This is explained in the Symfony2 Book, chapter on Doctrine
For your case, I would suggest using the findBy() method.
From the official doctrine documentation:
function getUserFromGroup($group, $criteria, $limit, $offset){
// You should probably build the criteria into a paramaters array,
// but I'll just asume it's "fieldName" => "valueToFilterBy"
$criteria['groups'] = $group;
$users = $em->getRepository('AppBundle\Entity\User')
->findBy(
$criteria, // Filter by columns
array('name' => 'ASC'), // Sorting
$limit, // How many entries to select
$offset // Offset
);
return $users;
};

Answer 2

I would not use the associations to list group's members, but a custom repository call. This should be close enough:
class PersonRepository extends EntityRepository
{
public function findPeopleInGroup(Group $group, $criteria, $limit, $offset){
$qb = $this->createQueryBuilder('p');
$qb->join('p.groups', 'g')
->where(':group MEMBER OF p.groups')
->setParameter('group', $group)
->orderBy('p.'.$criteria);
$qb->setFirstResult($offset);
$qb->setMaxResults($limit);
return $qb->getQuery()->getResult();
}
}