Question

Hello stackoverflow社区。

我想知道，如果OpenJPA中有可能在插入之前首先检查对象是否已存在于数据库中。

我有两个分支：

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;
    }

如果我想添加新的播放器或团队，JPA应首先检查此播放器或团队是否已在数据库中。如果是，那么它应该使用现有单元的主/外键。

这可能吗？或者我需要编写自己的方法吗？

最诚挚的问候 Veote

Answer 1

您可能需要检查此开关： http://openjpa.apache.org/builds/2.2.0/apache-openjpa/docs/jpa_2.2.html#jpa_2.2_cascadePersist

我猜您使用的是OpenJPA 2.2.0。

Answer 2

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);
    . . . 
}

如果您调用EntityManager.persist（...）并传入DB中已存在的Player实例，则JPA（根据规范）将抛出EntityExistsException。我建议您阅读EntityManager javadoc，以便更好地了解您可以使用API执行的操作。另外，我建议在实体中添加@Version字段。

Answer 3

是的我认为你可以... http://openjpa.apache.org/builds/1.2.2/apache-openjpa-1.2.2/docs/manual/manual.html#ref_guide_integration_revmappingtool访问此网站了解更多详情..

Apache OpenJPA - 仅在数据库中不存在时才插入元组

3 个答案: