一對多單向關聯

多的一方:不寫任何關聯關系

package com.eneitys;
 

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
 

@Entity
@Table(name="tb_student")
public class Student {
    
    private int id;
    private String stuName;
    
    @Id
    @GeneratedValue
    public int getId() {
       return id;
    }
    public void setId(int id) {
       this.id = id;
    }
    public String getStuName() {
       return stuName;
    }
    public void setStuName(String stuName) {
       this.stuName = stuName;
    }
}
           

一的一方:添加多的一方的Set集合

package com.eneitys;
 

import java.util.HashSet;
import java.util.Set;
 

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;
 

@Entity
@Table(name="tb_group")
public class Group {
    
    private int id;
    private String grpName;
    
    private Set<Student> set=new HashSet<Student>();
    
    @Id
    @GeneratedValue
    public int getId() {
       return id;
    }
    public void setId(int id) {
       this.id = id;
    }
    public String getGrpName() {
       return grpName;
    }
    public void setGrpName(String grpName) {
       this.grpName = grpName;
    }
    
    @OneToMany
    @JoinColumn(name="group_id")
    public Set<Student> getSet() {
       return set;
    }
    public void setSet(Set<Student> set) {
       this.set = set;
    }
}
           

總結:

@OneToMany在預設情況下,Hibernate會把一對多當成多對多的一種特殊情況!結果就是會幫我們生成一張中間表,是以注意要寫@JoinColumn(name="")注解,因為在資料庫中,不管是一對多還是多對一,都是在多的一方添加一的一個外鍵,是以這個jioncolumn的name值一般寫成一的外鍵字段名