mybatis一对多多对一多对多

左手的ㄟ右手 2022-06-01 00:54 310阅读 0赞

**MyBatis中的一对多和对多一，多对多**  
主要就是resultMap中  
**association** – 一个复杂的类型关联;许多结果将包成这种类型（一对多）  
**collection** – 复杂类型的集合（多对一）  
这2个属性的使用，而一对多和多对一都是相互的，只是站的角度不同。

**实例：**  
这个实例只说这2个属性的使用方法，具体的配置和运行结果在[SSM环境搭建][SSM]文章中写出。  
1.首先是数据库，沿用之前员工管理系统的数据库设计[点击这里][Link 1]，上面是之前写过的一篇数据库设计，包含sql语句。

2.实体类：  
我这里只列要实现映射功能用到的类user，role，department。department和user，是1对多关系，1个部门拥有多个员工。  
role和user是多对多关系，1个角色可能有多个员工，而1个员工也可能有多个角色。而要实现多对多，在程序中则是拆分成2个一对多，详情见下面的实体类注释。

1）User.java

public class User { 
        private int user_id;
        private String user_name;
        private String user_gender;
        private String user_email;
        private String user_phone;
        private String user_address;
        private Date user_birthday;
        private int department_id;
    
        //一对多，1个用户可能有多个角色。
        private List<Role> roles;
    
        //多对一，1个部门多个员工
        private Department department;
    
        public User() {
            super();
        }
    
        public User(String user_name, String user_gender,
                String user_email, String user_phone, String user_address,
                Date user_birthday, int department_id) {
            super();
            this.user_name = user_name;
            this.user_gender = user_gender;
            this.user_email = user_email;
            this.user_phone = user_phone;
            this.user_address = user_address;
            this.user_birthday = user_birthday;
            this.department_id = department_id;
        }
    
        public int getUser_id() {
            return user_id;
        }
    
        public void setUser_id(int user_id) {
            this.user_id = user_id;
        }
    
        public String getUser_name() {
            return user_name;
        }
    
        public void setUser_name(String user_name) {
            this.user_name = user_name;
        }
    
        public String getUser_gender() {
            return user_gender;
        }
    
        public void setUser_gender(String user_gender) {
            this.user_gender = user_gender;
        }
    
        public String getUser_email() {
            return user_email;
        }
    
        public void setUser_email(String user_email) {
            this.user_email = user_email;
        }
    
        public String getUser_phone() {
            return user_phone;
        }
    
        public void setUser_phone(String user_phone) {
            this.user_phone = user_phone;
        }
    
        public String getUser_address() {
            return user_address;
        }
    
        public void setUser_address(String user_address) {
            this.user_address = user_address;
        }
    
        public Date getUser_birthday() {
            return user_birthday;
        }
    
        public void setUser_birthday(Date user_birthday) {
            this.user_birthday = user_birthday;
        }
    
        public int getDepartment_id() {
            return department_id;
        }
    
        public void setDepartment_id(int department_id) {
            this.department_id = department_id;
        }
    
        public Department getDepartment() {
            return department;
        }
    
        public void setDepartment(Department department) {
            this.department = department;
        }
    
        public List<Role> getRoles() {
            return roles;
        }
    
        public void setRoles(List<Role> roles) {
            this.roles = roles;
        }
    
        @Override
        public String toString() {
            return "User [user_id=" + user_id + ", user_name=" + user_name
                    + ", user_gender=" + user_gender + ", user_email=" + user_email
                    + ", user_phone=" + user_phone + ", user_address="
                    + user_address + ", user_birthday=" + user_birthday
                    + ", department_id=" + department_id + ", roles=" + roles
                    + ", department=" + department + "]\n";
        }
    }
      
       
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2）Role.java

public class Role { 
        private int role_id;
        private String role_name;
    
        //一对多，1个角色也可能被多个用户拥有,所以是list
        private List<User> users;
    
        public Role() {
    
        }
    
        public Role(int role_id, String role_name) {
            super();
            this.role_id = role_id;
            this.role_name = role_name;
        }
    
        public int getRole_id() {
            return role_id;
        }
    
        public void setRole_id(int role_id) {
            this.role_id = role_id;
        }
    
        public String getRole_name() {
            return role_name;
        }
    
        public void setRole_name(String role_name) {
            this.role_name = role_name;
        }
    
        public List<User> getUsers() {
            return users;
        }
    
        public void setUsers(List<User> users) {
            this.users = users;
        }
    
        @Override
        public String toString() {
            return "Role [role_id=" + role_id + ", role_name=" + role_name
                    + ", users=" + users + "]";
        }
    
    }
    
      
       
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3）Department.java

public class Department { 
        private int department_id;
        private String department_name;
    
        //1对多，1个部门多个用户
        private List<User> users;
    
        public Department() {
            super();
        }
    
        public Department(int department_id, String department_name,
                List<User> users) {
            super();
            this.department_id = department_id;
            this.department_name = department_name;
            this.users = users;
        }
    
        public int getDepartment_id() {
            return department_id;
        }
    
        public void setDepartment_id(int department_id) {
            this.department_id = department_id;
        }
    
        public String getDepartment_name() {
            return department_name;
        }
    
        public void setDepartment_name(String department_name) {
            this.department_name = department_name;
        }
    
        public List<User> getUsers() {
            return users;
        }
    
        public void setUsers(List<User> users) {
            this.users = users;
        }
    
        @Override
        public String toString() {
            return "Department [department_id=" + department_id
                    + ", department_name=" + department_name + ", users=" + users
                    + "]"+"\n";
        }
    }
      
       
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3.实体类的mapper文件  
1）UserDepartmentMapper.xml，多对一，多个user对应1个department，使用association

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="employee_management.mapper.UserDepartmentMapper">
        <resultMap type="employee_management.entity.User" id="userDepartmentList">
            <id property="user_id" column="user_id" javaType="java.lang.Integer" />
            <result property="user_name" column="user_name" javaType="java.lang.String" />
            <result property="user_gender" column="user_gender" javaType="java.lang.String" />
            <result property="user_email" column="user_email" javaType="java.lang.String" />
            <result property="user_phone" column="user_phone" javaType="java.lang.String" />
            <result property="user_address" column="user_address" javaType="java.lang.String" />
            <result property="user_birthday" column="user_birthday" javaType="java.util.Date" />
            <result property="department_id" column="department_id" javaType="java.lang.Integer" />
                
            <association property="department" javaType="employee_management.entity.Department">
                <id property="department_id" column="department_id" javaType="java.lang.Integer" />
                <result property="department_name" column="department_name" javaType="java.lang.String" />
            </association>
        </resultMap>
    
        <select id="getAll" resultMap="userDepartmentList">
            select u.*,d.department_name from user u left join department d on u.department_id=d.department_id;
        </select>
    </mapper>
      
       
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2）UserMapper.xml，一对多，使用collection，1个user对应多个role，同时也是多对多中user这一方的一对多。如果要实现多对多，还需要完成1个role对应多个user,即完成2个1对多，写法同这里，就不在赘述。

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="employee_management.mapper.UserMapper">
    
        
        <resultMap type="employee_management.entity.User" id="userList">
            <id property="user_id" column="user_id" javaType="java.lang.Integer" />
            <result property="user_name" column="user_name" javaType="java.lang.String" />
            <result property="user_gender" column="user_gender" javaType="java.lang.String" />
            <result property="user_email" column="user_email" javaType="java.lang.String" />
            <result property="user_phone" column="user_phone" javaType="java.lang.String" />
            <result property="user_address" column="user_address" javaType="java.lang.String" />
            <result property="user_birthday" column="user_birthday" javaType="java.util.Date" />
            <result property="department_id" column="department_id" javaType="java.lang.Integer" />
        </resultMap>
    
        
        <resultMap type="employee_management.entity.User" id="userRoleList" extends="userList">
            
            <collection property="roles" ofType="employee_management.entity.Role">
                <id property="role_id" column="role_id" javaType="java.lang.Integer" />
                <result property="role_name" column="role_name" javaType="java.lang.String" />
            </collection>
        </resultMap>
    
        </resultMap>
    
    <select id="getAll" resultMap="userList">
            select * from user
        </select>
    <select id="getAllUserAndRole" resultMap="userRoleList">
            select u.*,r.* from
            user u left join user_role ur on u.user_id=ur.user_id
            left join role r
            on r.role_id=ur.role_id;
        </select>
      
       
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不管是一对多还是多对一，还是多对多，只需要知道这2个属性哪个代表多哪个代表1，就可以很好的在实体的mapper文件中配置出来。

[SSM]: http://blog.csdn.net/u010857795/article/details/73512299
[Link 1]: http://blog.csdn.net/u010857795/article/details/55211731