Question

我想我有：

表编辑（id，business_name，增值税）
表作者（ID，姓名，电子邮件）
表图书（ID，标题，说明，fk_author，fk_editor）。

假设 fk_editor 是表格编辑器的id字段的外键， fk_author 是表格作者的id字段的外键。

假设关系书籍：作者 n：1 且书籍：编辑 n：1

问题是：如何加入三个表通过书籍表？这意味着，我必须在Book类中添加什么代码才能让Hibernate理解为与作者和编辑者建立关系？考虑一下我在作者和编辑器中有相同的id字段名称。这是我需要纠正的示例代码：

表作者：

package com.bytecode.jpaexample.SpringBootMySqlJpaRestExample;

import javax.persistence.*;
import java.io.Serializable;

@Entity
@Table(name = "authors")
public class Author implements Serializable{

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   @Column(name = "id")
   @OneToMany(fetch = FetchType.LAZY)
   private int id;

   @Column(name = "name")
   private String name;

   @Column(name = "surname")
   private String surname;

   @Column(name = "email")
   private String email;

   /* constructors and getters and setters omitted intentionally */

}

表编辑：

package com.bytecode.jpaexample.SpringBootMySqlJpaRestExample;

import javax.persistence.*;
import java.io.Serializable;

@Entity
@Table(name = "editors")
public class Editor implements Serializable{

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   @Column(name = "id")
   @OneToMany(fetch = FetchType.LAZY)
   private int id;

   @Column(name = "business_name")
   private String businessName;

   @Column(name = "vat")
   private String vat;

   /* constructors and getters and setters omitted intentionally */

}

表图书：

package com.bytecode.jpaexample.SpringBootMySqlJpaRestExample;

import javax.persistence.*;
import java.io.Serializable;

@Entity
@Table(name="books")
public class Book implements Serializable{

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   @Column(name = "id")
   private int id;

   @Column(name = "title")
   private String title;

   @Column(name = "description")
   private String description;

   @ManyToOne(fetch = FetchType.LAZY)      
   @JoinColumn(name = "id") //authors.id                
   @Column(name = "fk_author")
   private int fk_editor;

   @ManyToOne(fetch = FetchType.LAZY)      
   @JoinColumn(name = "id") //editors.id                
   @Column(name = "fk_editor")
   private int fk_editor;

   /* constructors and getters and setters omitted intentionally */
}

Answer 1

有几件事：

Author和Editor类不应将id字段注明为@OneToMany：

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private int id;

相反，我猜你想在每个班级中想要另一个领域：

@Entity
@Table(name = "authors")
public class Author implements Serializable {
    ...
    @OneToMany(mappedBy = "author", fetch = FetchType.LAZY)
    private List<Book> books;
    ...
}

@Entity
@Table(name = "editors")
public class Editor implements Serializable {
    ...
    @OneToMany(mappedBy = "editor", fetch = FetchType.LAZY)
    private List <Book> books;
    ...
}

然后，您需要以多种方式更改班级Book：

更恰当地重命名字段（例如fk_author变为author）;这些必须与相应类中的mappedBy设置匹配，如上所述）
将字段类型更改为相应的类（例如Author），不外键类型（例如int）
更改@JoinColumn注释以指定books表中的相应列（不目标表）并添加referencedColumnName设置（尽管这些是在这种情况下是可选的;因为它们将默认为目标实体上的单个主键字段）
删除@Column注释

看看这是否有意义：

@Entity
@Table(name="books")
public class Book implements Serializable {
    ...
    @ManyToOne(fetch = FetchType.LAZY)      
    @JoinColumn(name = "fk_author", referencedColumnName="id")
    private Author author;

    @ManyToOne(fetch = FetchType.LAZY)      
    @JoinColumn(name = "fk_editor", referencedColumnName="id")
    private Editor editor;
    ...
}

JPA连接具有相同名称的外键

1 个答案: