一、概述

MyBatis 中的一对多、多对一和多对多，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。

首先是数据库，以员工管理系统的数据库为例。
实体类：
这里只列要实现映射功能用到的类 user、role、department。

department 和 user，是一对多关系，一个部门可以拥有多个员工。
role和user是多对多关系，一个角色可能有多个员工，而一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两个一对多，详情见下面的实体类注释。
1）User.java

@Data
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，一个用户可能有多个角色。
    private List<Role> roles;

    //多对一，一个部门多个员工
    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;
    }
}

2）Role.java

@Data
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 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 + "]";
    }
}

3）Department.java

@Data
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 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";
    }
}

3.实体类的 mapper 文件
1）UserDepartmentMapper.xml：多对一，多个 user 对应一个 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" />
            <!-- property是user实体类中department类属性的名字
            多个user对应一个department-->
        <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>

2）UserMapper.xml：一对多，使用collection，一个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，还需要完成一个role对应多个user，即完成两个一对多，写法同下。

<?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">

    <!-- get user list only -->
    <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>

    <!-- get user list with role -->
    <resultMap type="employee_management.entity.User" id="userRoleList"
        extends="userList">
        <!-- collection中不是javaType而是ofType,通过property也知道是list，即多个。
        extends属性，即在父resultMap的基础上再加上子resultMap的结果，在这里即再加上部门的信息。 -->
        <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>