mybatis处理一对多的映射关系

实体类

package org.example.entity;
import java.util.List;

public class Dept {
    private Integer deptId;
    private String deptName;
    private List<Emp> emps;

    public Dept() {
    }
    public Dept(Integer deptId, String deptName) {
        this.deptId = deptId;
        this.deptName = deptName;
    }

    public Integer getDeptId() {
        return deptId;
    }

    public void setDeptId(Integer deptId) {
        this.deptId = deptId;
    }

    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    public List<Emp> getEmps() {
        return emps;
    }

    public void setEmps(List<Emp> emps) {
        this.emps = emps;
    }

    @Override
    public String toString() {
        return "Dept{" +
                "deptId=" + deptId +
                ", deptName='" + deptName + '\'' +
                ", emps=" + emps +
                '}';
    }
}

mapper接口

 Dept getEmpAndDeptByDeptId(@Param("deptId") int deptId);

方式一：

collection 标签

<?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="org.example.mapper.DeptMapper"><resultMap id="empAndDeptResultMap" type="Dept">
        <id column="dept_id" property="deptId"></id>
        <result column="dept_name" property="deptName"></result>

        <!--
            collection：处理一对多的映射关系（处理集合类型的属性）
            ofType：设置集合类型的属性中存储的数据的类型
        -->
        <collection property="emps" ofType="Emp">
            <id column="emp_id" property="empId"></id>
            <result column="emp_name" property="empName"></result>
            <result column="age" property="age"></result>
            <result column="gender" property="gender"></result>
        </collection>
    </resultMap>

    <select id="getEmpAndDeptByDeptId" resultMap="empAndDeptResultMap">
        select * from t_dept a left join t_emp b on a.dept_id = b.dept_id where a.dept_id = #{deptId}
    </select>

</mapper>

方式二：

分布查询

DeptMapper接口

Dept getDeptAndEmpStepOne(@Param("deptId") int deptId);

EmpMapper接口

List<Emp> getDeptAndEmpStepTwo(@Param("deptId") Integer deptId);

DeptMapper.xml

<resultMap id="DeptAndEmpResultMapStepOne" type="Dept">
        <id column="dept_id" property="deptId"></id>
        <result column="dept_name" property="deptName"></result>

        <collection property="emps"
                    select="org.example.mapper.EmpMapper.getDeptAndEmpStepTwo"
                    column="dept_id">
        </collection>
    </resultMap>

    <select id="getDeptAndEmpStepOne" resultMap="DeptAndEmpResultMapStepOne">
        select * from t_dept where dept_id = #{deptId}
    </select>

EmpMapper.xml

 <select id="getDeptAndEmpStepTwo" resultType="org.example.entity.Emp">
        select  * from t_emp where dept_id = #{deptId}
 </select>

测试代码

@Test
    public void testGetDeptAndEmpByStep(){
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        DeptMapper mapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = mapper.getDeptAndEmpStepOne(1);
        System.out.println(dept);
        sqlSession.close();
    }

结果：

Dept{deptId=1, deptName='A', emps=[Emp{empId=1, empName='张三', age=10, gender='男', dept=null}, Emp{empId=4, empName='赵六', age=15, gender='男', dept=null}]}