Chapter 4 Mybatis-many multi-table queries of
4.1 User-many to achieve Role
By the previous study, we used Mybatis achieve the maintenance-many relationship. In fact, we have seen many relationships are two-way-to-many relationship.
4.1.1 relational model and user roles
4.1.2 business requirements and implement SQL
Requirements:
to achieve all of the objects and query load user information it assigned.
Analysis: The
query role we need to use the Role table, but the role assignment of user information and we can not find the information directly to the user, but can be associated with user information through the middle of the table (USER_ROLE table).
Here is the SQL statement:
SELECT
r.*,u.id uid,
u.username username,
u.birthday birthday,
u.sex sex,
u.address address
FROM
ROLE r
INNER JOIN
USER_ROLE ur
ON ( r.id = ur.rid)
INNER JOIN
USER u
ON (ur.uid = u.id);
4.1.3 Role write entity class
public class Role implements Serializable {
private Integer roleId;
private String roleName;
private String roleDesc;
//多对多的关系映射：一个角色可以赋予多个用户
private List<User> users;
public List<User> getUsers() {
return users;
}
public void setUsers(List<User> users) {
this.users = users;
}
public Integer getRoleId() {
return roleId;
}
public void setRoleId(Integer roleId) {
this.roleId = roleId;
}
public String getRoleName() {
return roleName;
}
public void setRoleName(String roleName) {
this.roleName = roleName;
}
public String getRoleDesc() {
return roleDesc;
}
public void setRoleDesc(String roleDesc) {
this.roleDesc = roleDesc;
}
@Override
public String toString() {
return "Role{" +
"roleId=" + roleId +
", roleName='" + roleName + '\'' +
", roleDesc='" + roleDesc + '\'' +
'}';
}
}
4.1.4 Role write persistence layer interface
public interface IRoleDao {
/**
* 查询所有角色
* @return
*/
List<Role> findAll();
}
4.1.5 write mapping file
<?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="com.itheima.dao.IRoleDao">
<!--定义role表的ResultMap-->
<resultMap id="roleMap" type="role">
<id property="roleId" column="rid"></id>
<result property="roleName" column="role_name"></result>
<result property="roleDesc" column="role_desc"></result>
<collection property="users" ofType="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="address" property="address"></result>
<result column="sex" property="sex"></result>
<result column="birthday" property="birthday"></result>
</collection>
</resultMap>
<!--查询所有-->
<select id="findAll" resultMap="roleMap">
select u.*,r.id as rid,r.role_name,r.role_desc from role r
left outer join user_role ur on r.id = ur.rid
left outer join user u on u.id = ur.uid
</select>
</mapper>
4.1.6 write test classes
public class RoleTest {
private InputStream in;
private SqlSession sqlSession;
private IRoleDao roleDao;
@Before//用于在测试方法执行之前执行
public void init()throws Exception{
//1.读取配置文件，生成字节输入流
in = Resources.getResourceAsStream("SqlMapConfig.xml");
//2.获取SqlSessionFactory
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
//3.获取SqlSession对象
sqlSession = factory.openSession(true);
//4.获取dao的代理对象
roleDao = sqlSession.getMapper(IRoleDao.class);
}
@After//用于在测试方法执行之后执行
public void destroy()throws Exception{
//提交事务
// sqlSession.commit();
//6.释放资源
sqlSession.close();
in.close();
}
/**
* 测试查询所有
*/
@Test
public void testFindAll(){
List<Role> roles = roleDao.findAll();
for(Role role : roles){
System.out.println("---每个角色的信息----");
System.out.println(role);
System.out.println(role.getUsers());
}
}
}
4.2 Role of the many-to achieve User
4.2.1 User to Role-many
From User, we can also find a user may have multiple roles, so that the user's relationship to the role still many relationship. So that we can think of many relationships User and Role, it can be broken down into two-to-many relationship to achieve.
