@OneToMany(targetEntity=Game.class, fetch=FetchType.LAZY,cascade=CascadeType.ALL)
@JoinColumn(name="SEASON_ID")
Collection<Game> games = new ArrayList<Game>();

@ManyToOne(targetEntity=Season.class, fetch=FetchType.LAZY)
Season season;

@OneToMany(targetEntity=GameSet.class, fetch=FetchType.LAZY,cascade=CascadeType.ALL)
@JoinColumn(name="GAME_ID")
Collection<GameSet> gameSets = new ArrayList<Game>();

@ManyToOne(targetEntity=Game.class, fetch=FetchType.LAZY)
Game game;

Caused by: java.sql.BatchUpdateException: Cannot add or update a child row: a foreign key constraint fails (`pingpong/game_set`, CONSTRAINT `FK9A6C52B5E6AFC091` FOREIGN KEY (`game_ID`) REFERENCES `season` (`ID`))
   at com.mysql.jdbc.PreparedStatement.executeBatchSerially(PreparedStatement.java:1669)
   at com.mysql.jdbc.PreparedStatement.executeBatch(PreparedStatement.java:1085)
   at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:70)
   at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:268)

   public Season createSeason(Contestant c, Contestant c2, Integer nbrGamesVsContestant, Integer setsPerGame,String name){
      
      Season season = new Season();
      //create a new list of Games
      ArrayList<Game> games = new ArrayList<Game>();
      
      Game game = new Game();
      game.setContestantOne(c);
      game.setContestantTwo(c2);
      game.setNbrOfSets(setsPerGame);
      
      //create a new list of GameSets
      ArrayList<GameSet> gameSets = new ArrayList<GameSet>();
      GameSet gs = new GameSet(0 , 0);
      
      //Add the the created GameSet to the list
      gameSets.add(gs);
      //Set the list of GameSet to the game
      game.setGameSets(gameSets);
      
      //add the game to the list of games
      games.add(game);
      
      //Set games and the rest of the values to season
      season.setGames(games);
      season.setGamesVsContestant(nbrGamesVsContestant);
      season.setName(name);
      season.setSetsPerGame(setsPerGame);

      return season;
   }

   public String execute(){

      SeasonCreator sc = new SeasonCreator();
      EntityManager em = ConnectDB.getEntityManager();
      try{
         em.getTransaction().begin();
         
         Contestant c = new Contestant();
         c.setFirstName("John");
         c.setLastName("Doe");
         
         Contestant c2 = new Contestant();
         c2.setFirstName("Julia");
         c2.setLastName("Doe");
         
         Season season = sc.createSeason(c, c2, 1, 1, "My first season");
         
         em.persist(season);

         em.getTransaction().commit();
      }catch (Exception e) {
         e.printStackTrace();
         em.getTransaction().rollback();
      }finally{
         if(em.isOpen()){
            em.close();
         }
      }
      
      return SUCCESS;
   }

game.setSeason(season);
season.getGames().add(game);