java Hibernate一对多映射覆盖

Question

在使用hibernate进行一对多映射时，我面临着一个hibernate问题。下面是我的两个实体类和联接表实体类。 ArticleCategoryMap。爪哇

    @Entity
    @Table(name = "ARTICLECATEGORYMAP")
    public class ArticleCategoryMap {
    private static final long serialVersionUID = -5653708523600543988L;

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column ( name = "id")
    Long id;


    @ManyToOne(targetEntity = Article.class, fetch = FetchType.EAGER, optional = true, cascade = CascadeType.PERSIST)
    @JoinColumn(name = "ARTICLE_ID", nullable = true, insertable = true, updatable = true)
    private Article article;


    @ManyToOne(targetEntity = Category.class, fetch = FetchType.EAGER, optional = true, cascade = CascadeType.PERSIST)
    @JoinColumn(name = "CATEGORY_ID", nullable = true, insertable = true, updatable = true)
    private Category category;

   //setter and getter  
}

文章。爪哇

    @Entity
    @Table(name = "ARTICLE")
    public class Article {
    private long id;
    private String title;
    private String description;
    private String keywords;
    private String content;

    @Id
    @GeneratedValue
    @Column(name = "ARTICLE_ID")
    public long getId() {
        return id;
    }
   //setter and getter  
    }

类别。爪哇

  @Entity
    @Table(name = "CATEGORY")
    public class Category {

    private long id;
    private String name;

    @OneToMany(cascade = CascadeType.ALL, fetch=FetchType.EAGER)
    @JoinTable(
            name = "ARTICLECATEGORYMAP",
            joinColumns = @JoinColumn(name = "CATEGORY_ID"),
            inverseJoinColumns = @JoinColumn(name = "ARTICLE_ID")
    )
    @CollectionId(
            columns = @Column(name="id"), 
            type=@Type(type="long"), 
            generator = "sequence"
    )
    private Collection<Article> articles;



    @Id
    @GeneratedValue
    @Column(name = "CATEGORY_ID")
    public long getId() {
        return id;
    }

    @OneToMany(cascade = CascadeType.ALL, fetch=FetchType.EAGER)
    @JoinTable(
            name = "ARTICLECATEGORYMAP",
            joinColumns = @JoinColumn(name = "CATEGORY_ID"),
            inverseJoinColumns = @JoinColumn(name = "ARTICLE_ID")
    )
    @CollectionId(
            columns = @Column(name="id"), 
            type=@Type(type="long"), 
            generator = "sequence"
    )
    // setter an getter
    }

现在假设我第一次在article表中有两个元素，它们映射到category表的一个条目。所以连接表看起来像

现在由于某种原因，我想更新条目，其中文章条目将映射到一个新的类别ID

所以我的问题是如何更新这个连接表

Answer 1

如果你想要一对多关系（一个类别有许多文章，一个类别有一个文章对一个类别），你不需要一个联接表

实体类应该如下所示：

类别实体：

包含一组文章：

   @Entity
    @Table(name = "CATEGORY")
    public class Category {

    private long id;
    private String name;

    @OneToMany(mappedBy="category")
        private Set<Article> articles;

          ......
    }

文章实体：

    @Entity
    @Table(name = "ARTICLE")
    public class Article {

    @ManyToOne
    @JoinColumn(name="id", nullable=false)
    private Category category;

    private long id;
    private String title;
    private String description;
    private String keywords;
    private String content;

    .......
    }

有关更多详细信息，请查看hibernate-one-to-many。希望这有帮助

还可以将注释从方法移动到字段。这是：

private long id;

@Id
@GeneratedValue
@Column(name = "CATEGORY_ID")
public long getId() {
    return id;
}

应该是：

    @Id
    @GeneratedValue
    @Column(name = "CATEGORY_ID")
    private long id;

   public long getId() {
        return id;
    }

多对多关系：

在数据库中有3个表：

1. CATEGORY
2. ARTICLE
3. ARTICLECATEGORYMAP (join table)

对于多对多关系实体而言：

类别实体：

    @Entity
        @Table(name = "CATEGORY")
        public class Category {

        @Id
        @GeneratedValue
        @Column(name = "CATEGORY_ID")
        private long id;
        private String name;

       @ManyToMany(cascade = { CascadeType.ALL })
       @JoinTable(
        name = "ARTICLECATEGORYMAP", 
        joinColumns = { @JoinColumn(name = "CATEGORY_ID") }, 
        inverseJoinColumns = { @JoinColumn(name = "ARTICLE_ID") }
    )
    Set<Article > articles = new HashSet<>();
     .....
   }

文章实体：

    @Entity
    @Table(name = "ARTICLE")
    public class Article {


    @Id
    @GeneratedValue
    @Column(name = "ARTICLE_ID") 
    private long id;
    private String title;
    private String description;
    private String keywords;
    private String content;

    @ManyToMany(mappedBy = "articles")
    private Set<Category> categories = new HashSet<>();

    .......
    }

有关更多信息，请查看many-to-many ralationship