Problem with one-to-many relation in MySQL

I see the issue — there are two separate problems in your code that explain why:

1. The foreign key is NULL when inserting a referee

2. Your query for referees by sport doesn’t work

1. Why the foreign key is NULL

When you save a Referee, you are not setting the sport field before calling save.
In TypeORM, the @ManyToOne side owns the foreign key, so you must explicitly assign a Sport (or at least its id) when saving.

// referee.service.ts
async add(refereeDto: { name: string, sportId: string }) {
  const sport = await this.sportsRepository.findOneBy({ id: refereeDto.sportId });
  if (!sport) throw new NotFoundException('Sport not found');

  const referee = this.refereesRepository.create({
    name: refereeDto.name,
    sport: sport,   // ? set relation here
  });

  return await this.refereesRepository.save(referee);
}

If you just call save(referee) without assigning .sport, the sportId column stays NULL.

2. Why you can’t read referees for a sport

In your findBySport method, you are filtering on the referee’s own id, not on its relation’s id.

async findBySport(sportId: string): Promise { 
return this.refereesRepository.find({ 
where: { sport: { id: sportId } }, // filter by relation 
relations: ['sport'], // optional: eager load sport 
}); }

 

Your current code:

where: { id: sportId }

… will only match referees whose own id equals the sport’s id — which never happens.

3. Controller fix

If your route looks like /sports/:sportId/referees, you need to pass sportId correctly:

@Get(':sportId/referees')
getRefereesBySport(@Param('sportId') sportId: string) {
  return this.refereesService.findBySport(sportId);
}

Summary of fixes

• In add(), assign the sport when saving a Referee.

• In findBySport(), filter by { sport: { id: sportId } }, not { id: sportId }.

• Ensure your controller route matches the parameter (:sportId).