mybatis处理多对一的映射关系
2024-09-08 19:41:32
创建数据库t_emp和t_dept
创建对应实体类
package org.example.entity;
public class Emp {
private Integer empId;
private String empName;
private Integer age;
private String gender;
private Dept dept;
public Emp() {
}
public Emp(Integer empId, String empName, Integer age, String gender, Dept dept) {
this.empId = empId;
this.empName = empName;
this.age = age;
this.gender = gender;
this.dept = dept;
}
public Integer getEmpId() {
return empId;
}
public void setEmpId(Integer empId) {
this.empId = empId;
}
public String getEmpName() {
return empName;
}
public void setEmpName(String empName) {
this.empName = empName;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public Dept getDept() {
return dept;
}
public void setDept(Dept dept) {
this.dept = dept;
}
@Override
public String toString() {
return "Emp{" +
"empId=" + empId +
", empName='" + empName + '\'' +
", age=" + age +
", gender='" + gender + '\'' +
", dept=" + dept +
'}';
}
}
package org.example.entity;
public class Dept {
private Integer deptId;
private String deptName;
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;
}
@Override
public String toString() {
return "Dept{" +
"deptId=" + deptId +
", deptName='" + deptName + '\'' +
'}';
}
}
mapper接口
public Emp getEmpAndDeptByEmpId(@Param("empId") Integer empId);
处理方式一：级联方式处理
mapper.xml
<resultMap id="empAndDeptResultMap" type="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>
<result column="dept_id" property="dept.deptId"></result>
<result column="dept_name" property="dept.deptName"></result>
</resultMap> <select id="getEmpAndDeptByEmpId" resultMap="empAndDeptResultMap">
SELECT t_emp.*,t_dept.dept_name FROM t_emp left join t_dept on t_emp.dept_id = t_dept.dept_id where t_emp.emp_id = #{empId}
</select>
处理方式二：使用association
mapper.xml
<resultMap id="empAndDeptResultMap" type="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>
<!--
association：处理多对一的映射关系（处理实体类类型的属性）
property：设置需要处理映射关系的属性的属性名
javaType：设置需要处理的属性的类型
-->
<association property="dept" javaType="Dept">
<id column="dept_id" property="deptId"></id>
<result column="dept_name" property="deptName"></result>
</association>
</resultMap> <select id="getEmpAndDeptByEmpId" resultMap="empAndDeptResultMap">
SELECT t_emp.*,t_dept.dept_name FROM t_emp left join t_dept on t_emp.dept_id = t_dept.dept_id where t_emp.emp_id = 1
</select>
测试代码
处理方式三：分步查询
创建EmpMapper
public interface EmpMapper {
public Emp getEmpAndDeptByStepOne(@Param("empId") Integer empId);
}
创建DeptMapper
public interface DeptMapper {
Dept getEmpAndDeptByStepTwo(@Param("deptId") Integer deptId);
}
EmpMapper.xml
<resultMap id="empAndDeptByStepResultMap" type="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>
<!--
property：设置需要处理映射关系的属性的属性名
select：设置分布查询的sql的唯一标识
column：将查询出的某个字段作为分布查询的sql的条件
-->
<association property="dept" column="dept_id" select="org.example.mapper.DeptMapper.getEmpAndDeptByStepTwo">
<id column="dept_id" property="deptId"></id>
<result column="dept_name" property="deptName"></result>
</association>
</resultMap> <select id="getEmpAndDeptByStepOne" resultMap="empAndDeptByStepResultMap">
select *
from t_emp where emp_id = #{empId};
</select>
DeptMapper.xml
<?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"> <select id="getEmpAndDeptByStepTwo" resultType="dept">
select * from t_dept where dept_id = #{deptId}
</select>
</mapper>
测试代码
@Test
public void testGetEmpAndDeptByStepOne(){
SqlSession sqlSession = SqlSessionUtil.getSqlSession();
EmpMapper mapper = sqlSession.getMapper(EmpMapper.class);
Emp emp = mapper.getEmpAndDeptByStepOne(2);
System.out.println(emp);
sqlSession.close();
}
mybatis懒加载全局配置
<settings>
<!--标准的日志-->
<setting name="logImpl" value="STDOUT_LOGGING"/>
<setting name="mapUnderscoreToCamelCase" value="true"/>
<!--开启懒加载(开启延迟加载)-->
<setting name="lazyLoadingEnabled" value="true"/>
<!--关闭实时加载-->
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
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