@ElementCollection 至少包含一個元素，最多包含 N 個元素

Question

有沒有辦法為@ElementCollection引用的實體應用引用計數約束？

簡化示例：

public enum Key {
    A, S, D;
};

@Entity
public class Keyboard {
     @Id
     Long id;

     // require at least one key?
     // require at most N keys?
     @ElementCollection(targetClass = Key.class)
     @JoinTable(name = "keyboard_key", joinColumns = @JoinColumn(name = "keyboard_id"))
     @Column(name = "key", nullable = false)
     @Enumerated(EnumType.STRING)
     Set<Key> keys; 
} 

Keyboard必須至少有1 Key 。 最多有N Key也很好。

考慮KeyboardRepository ：

public interface KeyboardRepository extends JpaRepository<Keyboard, Long> {}

//...
@Autowired
KeyboardRepository repo;

//..
repo.save(new Keyboard(/*id*/null, /*keys*/null); // should throw due to no-keys
repo.save(new Keyboard(/*id*/null, /*keys*/Sets.newSet()); // should throw due to no-keys
repo.save(new Keyboard(/*id*/null, /*keys*/Sets.newSet(Key.A, Key.S, Key.D)); // should throw due to eg. at most 2 keys

Answer 1

嘗試像這樣使用javax.validation.constraints.Size注釋：

@ElementCollection(targetClass = Key.class)
     @JoinTable(name = "keyboard_key", joinColumns = @JoinColumn(name = "keyboard_id"))
     @Column(name = "key", nullable = false)
     @Enumerated(EnumType.STRING)
     @Size(min = 1, max = N)
     Set<Key> keys; 

有關更多詳細信息，請查看如何在 spring-boot https://www.baeldung.com/javax-validation中使用 java bean 驗證