java - JPA OneToMany 和 ManyToOne 之间的关系

Question

我是 JPA 新手，并尝试在两个表之间建立关系，如下所示:

这是主类对象XParserLinks:

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "link_id")
    private Integer linkId;
    @OneToMany(mappedBy = "xParserLink", targetEntity = XLinksMedia.class, cascade = CascadeType.ALL)
    private List<XLinksMedia> fotos;
    ...

这是对象类XLinksMedia

public class XLinksMedia implements Serializable {
    private static final long serialVersionUID = 1L;
    @EmbeddedId
    protected XLinksMediaPK xLinksMediaPK;
    @ManyToOne @JoinColumn(name = "link_id")
    private XParserLinks xParserLink;

和XLinksMediaPK

@Embeddable
public class XLinksMediaPK implements Serializable {
    @Basic(optional = false)
    @Column(name = "link_id", insertable = false, updatable = false)
    private int linkId;
    @Basic(optional = false)
    @Column(name = "image")
    private String image;

这些关系正确吗？因为当我想在 fotos 列表中添加新的 XLinksMedia 对象时，我得到 LinkId = 0，但我知道 LinkId 不是 0，我知道 LinkId 变量有一个值。

这是我尝试在 XParseLinks 类中设置 fotos 变量的方法(preke:是 XParseLinks 对象)他的主键 LinkId):

XLinksMedia foto = new XLinksMedia();
foto.setXLinksMediaPK(new XLinksMediaPK());

foto.setxParserLink(preke);
foto.getXLinksMediaPK().setImage(e.attr("src"));

preke.getFotos().add(foto);

Answer 1

即使我不知道你的期望是什么，当我测试你的编码时，它是好的。我只是在您的类中放置了一些OverLoad Constructor，以便轻松进行数据初始化。但是，您使用 GenerationType.IDENTITY，这就是我为 XParserLinks 的 link_id 设置自动创建列的原因。以下代码的主要内容与您的相同。

您的观点这是我尝试在 XParseLinks 类中设置 fotos 变量的方法(preke:是 XParseLinks 对象，它有他的主键 LinkId):。检查Test.java

XParserLinks.java

@Entity
public class XParserLinks implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "link_id")
    private Integer linkId;
    @OneToMany(mappedBy = "xParserLink", targetEntity = XLinksMedia.class, cascade = CascadeType.ALL)
    private List<XLinksMedia> fotos;

    private String something;

    public XParserLinks() {
        fotos = new ArrayList<XLinksMedia>();
    }

    public XParserLinks(String something) {
        this.something = something;
        fotos = new ArrayList<XLinksMedia>();
    }
    //getter and setter
}       

XLinksMediaPK.java

@Embeddable
public class XLinksMediaPK implements Serializable {
    @Basic(optional = false)
    @Column(name = "link_id", insertable = false, updatable = false)
    private int linkId;
    @Basic(optional = false)
    @Column(name = "image")
    private String image;

    public XLinksMediaPK(){
    }

    public XLinksMediaPK(int linkId, String image) {
        this.linkId = linkId;
        this.image = image;
    }
    //getter and setter
}

XParserLinks.java

@Entity
public class XParserLinks implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "link_id")
    private Integer linkId;
    @OneToMany(mappedBy = "xParserLink", targetEntity = XLinksMedia.class, cascade = CascadeType.ALL)
    private List<XLinksMedia> fotos;

    private String something;

    public XParserLinks() {
        fotos = new ArrayList<XLinksMedia>();
    }

    public XParserLinks(String something) {
        this.something = something;
        fotos = new ArrayList<XLinksMedia>();
    }
    //getter and setter

    public void add(XLinksMedia media) {
        fotos.add(media);
    }
}

测试.java

public class Test {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("JPA");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        XParserLinks xParserLink = new XParserLinks("something");

        XLinksMediaPK pk1 = new XLinksMediaPK(0, "image_1");
        XLinksMediaPK pk2 = new XLinksMediaPK(0, "image_2");
        XLinksMediaPK pk3 = new XLinksMediaPK(0, "image_3");

        xParserLink.add(new XLinksMedia(pk1, xParserLink));
        xParserLink.add(new XLinksMedia(pk2, xParserLink));
        xParserLink.add(new XLinksMedia(pk3, xParserLink));

        em.persist(xParserLink);

        System.out.println("Success...");

        em.getTransaction().commit();
        em.close();
    }
}