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"; } }
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>