@Entity
class Schedule {...}

@Entity
class Activity {
   @Id
   private Long id;

   @Getter
   @OneToMany(cascade = CascadeType.ALL)
   private Set<Schedule> schedule;

   @Getter
   @OneToMany(fetch=FetchType.LAZY)
   @Fetch(FetchMode.SELECT)
   @BatchSize(size=15)
   @JoinFormula(value = "SELECT s FROM Schedule s WHERE activity = id AND start > NOW()")
   private List<Schedule> futureSchedule;

   ...
}