Apache OpenJPA - 僅當數據庫中不存在時才插入元組

Question

你好，stackoverflow社區。

我想知道，如果OpenJPA有可能首先檢查對象在插入前是否已經存在於數據庫中。

我有兩個clases：

imports... 

@Entity(name="Player") 
public class Player { 
    private long id; 
    private String nickName; 
    private Team team; /* A Player is on a Team */ 

    @ManyToOne(cascade=CascadeType.PERSIST) 
    @JoinColumn(name = "team_id") 
    public Team getTeam() { 
     return team; 
    } 

    @Id 
    @GeneratedValue(strategy=GenerationType.IDENTITY) 
    public long getId() { 
     return id; 
    } 

    /* more getters/setters ...*/ 
} 
@Entity(name="Team") 
public class Team { 
    private long id; 
    private String name; 
    private List<Player> players; /* A Team Has Players */ 

    @OneToMany(mappedBy = "team", targetEntity = Player.class, 
      fetch = FetchType.EAGER, cascade = CascadeType.PERSIST) 
    public List<Player> getPlayers() { 
     return players; 
    } 

    @Id 
    @GeneratedValue(strategy=GenerationType.IDENTITY) 
    public long getId() { 
     return id; 
    } 

如果我想添加一個新的球員或球隊，JPA應該先檢查是否該球員或球隊已經在數據庫中。 如果是，那麼它應該使用現有單元的主鍵/外鍵。

這可能嗎？或者我需要寫我自己的方法？

問候 Veote

Answer 1

你可能要檢查這個開關： http://openjpa.apache.org/builds/2.2.0/apache-openjpa/docs/jpa_2.2.html#jpa_2.2_cascadePersist

我猜你正在使用的OpenJPA 2.2.0。

Answer 2

是的，我想你可以... http://openjpa.apache.org/builds/1.2.2/apache-openjpa-1.2.2/docs/manual/manual.html#ref_guide_integration_revmappingtool去這個網站了解更多詳情..

Answer 3

public interface EntityManager { 
/** 
* Make an instance managed and persistent. 
* @param entity 
* @throws EntityExistsException if the entity already exists. 
* (If the entity already exists, the EntityExistsException may 
* be thrown when the persist operation is invoked, or the 
* EntityExistsException or another PersistenceException may be 
* thrown at flush or commit time.) 
* @throws IllegalArgumentException if the instance is not an 
* entity 
* @throws TransactionRequiredException if invoked on a 
* container-managed entity manager of type 
* PersistenceContextType.TRANSACTION and there is 
* no transaction 
*/ 
public void persist(Object entity); 
    . . . 
} 

如果調用EntityManager.persist（...），並通過在一個已經存在於數據庫中的Player實例，JPA（根據規範）將拋出一個EntityExistsException異常。我建議您閱讀EntityManager javadoc以更好地瞭解您可以使用API​​做什麼。另外，我建議在您的實體中添加一個@Version字段。