Symfony simple form filter question only allow unique entries - symfony

Question

I'm stuck for a while now in trying to get the following scenario to work.
I have a registration system for a sporting team.
Each team can register for any available contest. The problem is that each team is only allowed once to register on a contest. I simply want to filter the teams that are already registered for the current selected contest. I'm using the Symfony framework together with Doctrine ORM and the symfony form system.
I tried to create a custom function inside the teamRepository to only get the teams that are not registered for a given contest. But i can't seem to find a way to gt this to work.
I'm using 3 entities for this: User, Team, Registration, Contest
The registration entity holds all te submitted registrations with a relation to the Contest and the User (team). I hope that someone can help me here. For reference, this is my registration entity:
class Registrations
{
use TimestampableEntity;
#[ORM\Id]
#[ORM\GeneratedValue]
#[ORM\Column(type: 'integer')]
private $id;
#[ORM\ManyToOne(targetEntity: Team::class, inversedBy: 'registrations')]
private $Team;
#[ORM\ManyToMany(targetEntity: Dancers::class, inversedBy: 'registrations', fetch: 'EAGER')]
private $Dancers;
#[ORM\Column(type: 'text', nullable: true)]
private $Comments;
#[ORM\ManyToOne(targetEntity: Contest::class, fetch: 'EAGER', inversedBy: 'registrations')]
#[ORM\JoinColumn(nullable: false)]
private $Contest;
#[ORM\Column(type: 'string', length: 255)]
#[Gedmo\Blameable(on: 'create')]
private $RegisteredBy;
#[ORM\OneToMany(mappedBy: 'Registration', targetEntity: Orders::class)]
private $orders;
#[ORM\Column(type: 'string', length: 255, nullable: true)]
private $MusicFile;
public function __construct()
{
$this->Dancers = new ArrayCollection();
$this->orders = new ArrayCollection();
}
public function __toString()
{
return $this->id;
}
public function getId(): ?int
{
return $this->id;
}
public function getTeam(): ?Team
{
return $this->Team;
}
public function setTeam(?Team $Team): self
{
$this->Team = $Team;
return $this;
}
/**
* #return Collection|Dancers[]
*/
public function getDancers(): Collection
{
return $this->Dancers;
}
public function addDancer(Dancers $dancer): self
{
if (!$this->Dancers->contains($dancer)) {
$this->Dancers[] = $dancer;
}
return $this;
}
public function removeDancer(Dancers $dancer): self
{
$this->Dancers->removeElement($dancer);
return $this;
}
public function getComments(): ?string
{
return $this->Comments;
}
public function setComments(?string $Comments): self
{
$this->Comments = $Comments;
return $this;
}
public function getRegisteredBy(): ?string
{
return $this->RegisteredBy;
}
public function setRegisteredBy(?string $RegisteredBy): self
{
$this->RegisteredBy = $RegisteredBy;
return $this;
}
public function getContest(): ?Contest
{
return $this->Contest;
}
public function setContest(?Contest $Contest): self
{
$this->Contest = $Contest;
return $this;
}
/**
* #return Collection<int, Orders>
*/
public function getOrders(): Collection
{
return $this->orders;
}
public function addOrder(Orders $order): self
{
if (!$this->orders->contains($order)) {
$this->orders[] = $order;
$order->setRegistration($this);
}
return $this;
}
public function removeOrder(Orders $order): self
{
if ($this->orders->removeElement($order)) {
// set the owning side to null (unless already changed)
if ($order->getRegistration() === $this) {
$order->setRegistration(null);
}
}
return $this;
}
public function getMusicFile(): ?string
{
return $this->MusicFile;
}
public function setMusicFile(string $MusicFile): self
{
$this->MusicFile = $MusicFile;
return $this;
}
}
This is the repo function as mentioned:
public function getTeamsNotRegistered($organisation, $contest)
{
return $this->createQueryBuilder('t')
->leftJoin('t.Registrations', 'r')
->andWhere('t.Organisation = :org')
->andWhere('r.Contest = :contest')
->setParameter('org', $organisation)
->setParameter('contest', $contest)
->getQuery()
->getResult();
}