mybatis入门篇（六）：Mybatis高级查询

一、ResultMap的association与collection association与collection功能相似，区别是一对一与一对多，这里以association为例。

首先说明一下需求：经过员工ID获取员工信息，同时获取员工的角色，涉及到了员工信息表、角色表、还有两者的关联表。最简单的作法是写一个SQL语句，语句里写了三个表，互相关联进行查询，但这种方式存在问题：一、SQL语句不易维护 二、复用性不强 三、我只想获取用户的信息，不用角色信息时，查询了多余的信息，徒增消耗。

解决办法：一、对每一个实体都定义一个基础的ResultMap，根据须要继承基础ResultMap再添加自定义的属性 二、在ResultMap中使用association或者collection进行关联查询 三、使用延迟加载，须要角色信息时再去查询。

实际操做过程：

新建实体类User和Role

import java.util.Date;
public class Role {
    private Long id;
    private String roleName;
    private Integer enabled;
    private Long createBy;
    private Date createTime;
    private Long updateBy;
    private Date updateTime;

    ...
    getter和setter就省略了
    ...
}

import java.util.Date;
import java.util.List;
public class User {
    private Long id;
    private String userName;
    private String userPassword;
    private String userPhone;
    private String userEmail;
    private Date createTime;
    private Date updateTime;
    private byte[] headImg;
    private Role role;

    ...
    getter和setter省略了
    ...
}

而后编写Mapper.xml

首先先编写其中的字段映射部分BaseResultMap。

UserMapper.xml

<resultMap id="BaseResultMap" type="com.forest.owl.entity.User">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="user_name" jdbcType="VARCHAR" property="userName" />
    <result column="user_password" jdbcType="VARCHAR" property="userPassword" />
    <result column="user_phone" jdbcType="VARCHAR" property="userPhone" />
    <result column="user_email" jdbcType="VARCHAR" property="userEmail" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
    <result column="head_img" jdbcType="LONGVARBINARY" property="headImg" />
</resultMap>

RoleMapper.xml

<resultMap id="BaseResultMap" type="com.forest.owl.entity.Role">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="role_name" jdbcType="VARCHAR" property="roleName" />
    <result column="enabled" jdbcType="INTEGER" property="enabled" />
    <result column="create_by" jdbcType="BIGINT" property="createBy" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_by" jdbcType="BIGINT" property="updateBy" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
</resultMap>

编写RoleMapper.xml获取Role的接口和XML

import com.forest.owl.entity.Role;
public interface RoleMapper {
    Role selectByPrimaryKey(Long id);
}

RoleMapper.xml

<select id="selectByPrimaryKey" resultMap="BaseResultMap">
    select id, role_name, enabled, create_by, create_time, update_by, update_time
    from role
    where id = #{id,jdbcType=BIGINT}
</select>

而后编写UserMapper.xml

<resultMap id="UserAndRole" extends="BaseResultMap" type="com.forest.owl.entity.User">
    <association property="role" fetchType="lazy" column="{id=role_id}" select="com.forest.owl.mapper.RoleMapper.selectByPrimaryKey"/>
</resultMap>

<select id="selectUsersById" resultMap="UserAndRole">
    SELECT u.*, ur.role_id
    FROM user u
    INNER JOIN user_role ur on ur.user_id=u.id
    WHERE u.id=#{id}
</select>

能够看到，UserMapper的SQL语句中少了role表，这样SQL语句就简洁多了。须要注意，association中的fetchType=“lazy”选项，当执行getRole()的时候，才回去查询角色信息加载到查询结果中。若是用户仍是须要在触发某个方法时加载所有数据，则能够配置mybatis的配置选项lazyLoadTriggerMethods，按需加载。

对于3.4.1 及以前版本的mybatis用户，还须要配置mybatis-config.xml的一个选项：

<setting name="aggressiveLazyLoading" value="false"/>

测试：

@Test
public void UserMapperTest(){
    SqlSession sqlSession = getSqlSession();
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    List<User> userList = userMapper.selectUsersById((long) 2);
    System.out.println(userList.size());
    System.out.println("--------getRole---------");
    System.out.println(userList.get(0).getRole());
}

执行结果

collection的代码

import java.util.Date;
import java.util.List;
public class User {
    private Long id;
    private String userName;
    private String userPassword;
    private String userPhone;
    private String userEmail;
    private Date createTime;
    private Date updateTime;
    private byte[] headImg;
    private List<Role> roleList;
    ...
    getter和setter省略
    ...
}

<?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.forest.owl.mapper.UserMapper">
  <resultMap id="BaseResultMap" type="com.forest.owl.entity.User">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="user_name" jdbcType="VARCHAR" property="userName" />
    <result column="user_password" jdbcType="VARCHAR" property="userPassword" />
    <result column="user_phone" jdbcType="VARCHAR" property="userPhone" />
    <result column="user_email" jdbcType="VARCHAR" property="userEmail" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
    <result column="head_img" jdbcType="LONGVARBINARY" property="headImg" />
  </resultMap>

  <resultMap id="UserAndRole" extends="BaseResultMap" type="com.forest.owl.entity.User">
    <collection property="roleList" columnPrefix="role_" fetchType="lazy"
                resultMap="com.forest.owl.mapper.RoleMapper.BaseResultMap">
      <id property="id" column="id"/>
      <result column="role_name" property="roleName" />
      <result column="enabled" property="enabled" />
      <result column="create_by" property="createBy" />
      <result column="create_time" property="createTime" />
      <result column="update_by" property="updateBy" />
      <result column="update_time" property="updateTime" />
    </collection>
  </resultMap>

  <select id="selectUsersById" resultMap="UserAndRole">
    SELECT u.*,
    r.id role_id,
    r.role_name role_role_name,
    r.enabled role_enabled,
    r.create_by role_create_by,
    r.create_time role_create_time,
    r.update_by role_update_by,
    r.update_time role_update_time
    FROM user u
    INNER JOIN user_role ur on ur.user_id=u.id
    INNER JOIN role r on r.id=ur.role_id
    WHERE u.id=#{id}
  </select>
</mapper>

UserMapper接口

import com.forest.owl.entity.User;
public interface UserMapper {
    User selectUsersById(Long id);
}

@Test
public void UserMapperTest(){
    SqlSession sqlSession = getSqlSession();
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    User user = userMapper.selectUsersById((long) 2);
    System.out.println(user.getRoleList().get(0).getRoleName());
    System.out.println(user.getRoleList().get(1).getRoleName());
}