Finding an entity that contain only some others entities - sql

Question

I have an association like this :
Chatroom >----< User
So a Chatroom can contains multiple users, and a User can belong to multiple Chatrooms.
Now I want to select all the chatrooms that contains a couple of user, and only this couple.
I tried some solutions, like this one :
public function findByUsers($firstUser, $secondUser){
$qb = $this->createQueryBuilder('c');
$qb
->select('c')
->where('c.users LIKE :firstUser')
->andwhere('c.users LIKE :secondUser')
->setParameters(array(
'firstUser' => $firstUser,
'secondUser' => $secondUser
));
return $qb->getQuery()->getResult();
}
But It doesn't work and return me that kind of error :
[Semantical Error] line 0, col 52 near 'users LIKE :firstUser': Error: Invalid PathExpression. Must be a StateFieldPathExpression.
Some users encountering this error resolved it by adding IDENTITY before the query selector, but I don't understand how to apply it in my case.
So, did someone know how I can get all the chatrooms containing my couple of users ?
Thanks a lot !
EDIT : Adding the doctrine relation annotations
User.php
/**
*
* #ORM\ManyToMany(targetEntity="Chatroom", inversedBy="users")
* #ORM\JoinTable(name="chatrooms_users",
* joinColumns={#ORM\JoinColumn(name="user_id", referencedColumnName="id")},
* inverseJoinColumns={#ORM\JoinColumn(name="chatroom_id", referencedColumnName="id")}
* )
*/
private $chatrooms;
Chatroom.php :
/**
* #ORM\ManyToMany(targetEntity="User", mappedBy="chatrooms")
*/
private $users;

Answer 1

My final solution was :
public function findByUsers($ids)
{
$count = count($ids);
$qb = $this->createQueryBuilder('chatroom');
$qb->select('chatroom')
->join('chatroom.users', 'u')
->addSelect('COUNT(u) AS HIDDEN ucount')
->groupBy('chatroom.id')
->having('ucount = :count')
->setParameter('count', $count);
foreach ($ids as $key => $id) {
$qb->andWhere(':id' . $key . ' MEMBER OF chatroom.users')
->setParameter('id'.$key, (int) $id);
}
return $qb->getQuery()->getOneOrNullResult();
}
Pass an array of users id (or simply users with some modifications), and the function returns the list of chatrooms that contains only these users