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  • hibernate一对多多对一双向

    注意事项:一对多,多对一双向关联,在一的一方的多的getSet集合上的oneToMany上加上mappedBy。告诉hibernate由多的方一来维护关系。这也符合逻辑

    ,本来外键就是在加在多的一方。

    User类:

    package com.oracle.hibernate;
    
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.ManyToOne;
    
    @Entity
    public class User {
    
        private int id;
        private String name;
        private Group  group;
        
        @ManyToOne
        public Group getGroup() {
            return group;
        }
        public void setGroup(Group group) {
            this.group = group;
        }
        @Id
        @GeneratedValue
        public int getId() {
            return id;
        }
        public void setId(int id) {
            this.id = id;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        
    }

    Group类:

    package com.oracle.hibernate;
    
    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.OneToOne;
    import javax.persistence.Table;
    
    @Entity
    @Table(name="t_group")//group是mysql的关键字,换个名
    public class Group {
    
        private int id;
        private String name;
        //用set集合,不重复。因为表里的记录也不会重复
        private Set<User>  users = new HashSet<User>();
        
        //铁律:双向关系,必设mappedBy,
        //在oneTomany一方设mappedBy,告诉hibernate,在多的一方来维护关系,在Group类里就不用管我了,
        @OneToMany(mappedBy="group")
        public Set<User> getUsers() {
            return users;
        }
        public void setUsers(Set<User> users) {
            this.users = users;
        }
        @Id
        @GeneratedValue
        public int getId() {
            return id;
        }
        public void setId(int id) {
            this.id = id;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        
    }

    生成的表:

    hibernate建表语句:

    create table t_group (
    id integer not null auto_increment,
    name varchar(255),
    primary key (id)
    )

    create table User (
    id integer not null auto_increment,
    name varchar(255),
    group_id integer,
    primary key (id)
    )

    alter table User
    add index FK285FEB80FC033B (group_id),
    add constraint FK285FEB80FC033B
    foreign key (group_id)
    references t_group (id)

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  • 原文地址:https://www.cnblogs.com/lihaoyang/p/4917235.html
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