The page with the list of players of a tournament has a select field to add players. This list should be filtered and only contain the names of the people not already playing in the tournament.
There is already a prepared method that needs just to be implemented with a proper (DQL) query. See /module/Application/src/Application/Entity/PlayerRepository.php line 50, method \Application\Entity\PlayerRepository::getPlayersNotInTournament.
The page with the list of players of a tournament has a select field to add players. This list should be filtered and only contain the names of the people not already playing in the tournament.
There is already a prepared method that needs just to be implemented with a proper (DQL) query. See /module/Application/src/Application/Entity/PlayerRepository.php line 50, method \Application\Entity\PlayerRepository::getPlayersNotInTournament.