Question

考虑下表：

CREATE TABLE Participations
(
  roster_id INTEGER NOT NULL,
  round_id INTEGER NOT NULL,
  ordinal_nbr SMALLINT NOT NULL ,
  was_withdrawn BOOLEAN NOT NULL,
  PRIMARY KEY (roster_id, round_id, ordinal_nbr),
  CONSTRAINT participations_rosters_fk FOREIGN KEY (roster_id) REFERENCES Rosters (id),
  CONSTRAINT participations_groups_fk FOREIGN KEY (round_id, ordinal_nbr) REFERENCES Groups (round_id , ordinal_nbr)
)

这里是JPA 1.0 @IdClass实体类：

@Entity
@Table(name = "Participations")
@IdClass(value = ParticipationId.class)
public class Participation implements Serializable
{
    @Id
    @Column(name = "roster_id", insertable = false, updatable = false)
    private Integer rosterId;

    @Id
    @Column(name = "round_id", insertable = false, updatable = false)
    private Integer roundId;

    @Id
    @Column(name = "ordinal_nbr", insertable = false, updatable = false)
    private Integer ordinalNbr;

    @Column(name = "was_withdrawn")
    private Boolean wasWithdrawn;

    @ManyToOne
    @JoinColumn(name = "roster_id", referencedColumnName = "id")
    private Roster roster = null;

    @ManyToOne
    @JoinColumns(value = {@JoinColumn(name = "round_id", referencedColumnName = "round_id"), @JoinColumn(name = "ordinal_nbr", referencedColumnName = "ordinal_nbr")})
    private Group group = null;

    public Participation()
    {
    }

    public Integer getRosterId()
    {
        return rosterId;
    }

    public void setRosterId(Integer rosterId)
    {
        this.rosterId = rosterId;
    }

    public Integer getRoundId()
    {
        return roundId;
    }

    public void setRoundId(Integer roundId)
    {
        this.roundId = roundId;
    }

    public Integer getOrdinalNbr()
    {
        return ordinalNbr;
    }

    public void setOrdinalNbr(Integer ordinalNbr)
    {
        this.ordinalNbr = ordinalNbr;
    }

    public Boolean getWasWithdrawn()
    {
        return wasWithdrawn;
    }

    public void setWasWithdrawn(Boolean wasWithdrawn)
    {
        this.wasWithdrawn = wasWithdrawn;
    }

    public Roster getRoster()
    {
        return roster;
    }

    // ???
    public void setRoster(Roster roster)
    {
        this.roster = roster;
    }

    public Group getGroup()
    {
        return group;
    }

    // ???
    public void setGroup(Group group)
    {
        this.group = group;
    }

    ...
}

一般来说，如果关联设置器与冗余字段同步，那么这里是rosterId，roundId和ordinalNbr？：

    // ???
    public void setGroup(Group group)
    {
        this.group = group;
        this.roundId = group.getRoundId();
        this.ordinalNbr = group.getOrdinalNbr();
    }

由于

Answer 1

是的，它们应该保持同步。虽然因为它们是Id的一部分，所以你永远不应该改变它们，所以它实际上只是新对象的一个问题。

如果你不保持同步，那么对于一个新对象，它们将为null / 0，这可能不是很好。 JPA中没有任何魔法可以让你保持同步。

如果您从数据库中读取对象，那么它们将是粗略的同步，但您负责在内存中维护对象的状态，包括重复字段和双向映射。

如果您使用的是JPA 2.0，为什么还要复制Ids呢？您可以删除routersId和roundId，只需将@Id添加到@ManyToOnes。

将冗余列属性与JPA 1.0 @IdClass实现中的关联同步是一种最佳实践吗？

1 个答案: