MyBatis一对多，多对一，多对多

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，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";
    }
}

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 + "]";
    }

}

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";
    }
}<br /><br /><br /><br /><br /><br />

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" />
            <!-- property是user实体类中department类属性的名字
            多个user对应1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，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">

    <!-- 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>