mybatis关系映射(1对1,1对多,多对多)
1.映射(多)对一、(一)对一的关联关系
1).使用列的别名
①.若不关联数据表,则可以得到关联对象的id属性
②.若还希望得到关联对象的其它属性。则必须关联其它的数据表
1.创建表:
员工表:
DROP TABLE IF EXISTS `tbl_employee`;
CREATE TABLE `tbl_employee` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_name` varchar(255) DEFAULT NULL,//DEFAULT默认值
`gender` char(1) DEFAULT NULL,
`email` varchar(255) DEFAULT NULL,
`d_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_emp_dept` (`d_id`),
CONSTRAINT `fk_emp_dept` FOREIGN KEY (`d_id`) REFERENCES `tbl_dept` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
部门表:
CREATE TABLE tbl_dept(
id INT(11) PRIMARY KEY AUTO_INCREMENT,
dept_name VARCHAR(255)
)
2.创建相应的实体类和Mapper接口!
3.写关联的SQL语句
SELECT e.`id` id , e.`user_name` user_name, e.`gender` gender,e.`d_id` d_id,d.`id` did,d.`dept_name` dept_name
FROM `tbl_employee` e, tbl_dept d
WHERE e.`d_id` = d.`id` AND e.id = 1
4.在sql映射文件中写映射sql语句【联合查询:级联属性封装结果集】
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
<!-- 联合查询:级联属性封装结果集 --> <resultMap type="com.neuedu.entity.Employee" id="getEmployeeAndDeptMap"> <id column="id" property="id"/> <result column="user_name" property="userName"/> <result column="gender" property="gender"/> <result column="email" property="email"/> <result column="did" property="depart.id"/> <result column="dept_name" property="depart.deptName"/> </resultMap> <!-- public Employee getEmployeeAndDept(Integer id); --> <select id="getEmployeeAndDept" resultMap="getEmployeeAndDeptMap"> SELECT e.`id` id , e.`user_name` user_name, e.`gender` gender,e.`email` email,e.`d_id` d_id,d.`id` did,d.`dept_name` dept_name FROM `tbl_employee` e, tbl_dept d WHERE e.`d_id` = d.`id` AND e.id = #{id} </select>
注意:即使使用resultMap来映射,对于“对一”关联关系可以不使用association
5.编写测试用例
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
public class TestMyBatis {
private SqlSession openSession = null;
@Test
public void testGetEmployee(){
EmployeeMapperPlus mapper = openSession.getMapper(EmployeeMapperPlus.class);
Employee employee = mapper.getEmployeeAndDept(1);
System.out.println(employee);
}
@Before
public void testBefore() throws IOException{
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
openSession= sqlSessionFactory.openSession();
}
@After
public void testAfter(){
openSession.commit();
openSession.close();
}
}
方法二:【使用association来定义关联对象的规则,[比较正规的,推荐的方式]】
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
<!-- 联合查询:使用association封装结果集 --> <resultMap type="com.neuedu.entity.Employee" id="getEmployeeAndDeptMap"> <id column="id" property="id"/> <result column="user_name" property="userName"/> <result column="gender" property="gender"/> <result column="email" property="email"/> <!-- association可以指定联合的javaBean对象 property="depart":指定哪个属性是联合的对象 javaType:指定这个属性对象的类型【不能省略】 --> <association property="depart" javaType="com.neuedu.entity.Department"> <id column="did" property="id"/> <result column="dept_name" property="deptName"/> </association> </resultMap> <!-- public Employee getEmployeeAndDept(Integer id); --> <select id="getEmployeeAndDept" resultMap="getEmployeeAndDeptMap"> SELECT e.`id` id , e.`user_name` user_name, e.`gender` gender,e.`email` email,e.`d_id` d_id,d.`id` did,d.`dept_name` dept_name FROM `tbl_employee` e, tbl_dept d WHERE e.`d_id` = d.`id` AND e.id = #{id} </select>
方法三[上述结果相当于使用嵌套结果集的形式]【我们这里还可以使用Association进行分步查询】:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
<!-- 使用association进行分步查询 1.先按照员工id查询员工信息 2.根据查询员工信息中d_id值取部门表查出部门信息 3.部门设置到员工中: --> <select id="getDepartById" resultType="com.neuedu.entity.Department"> SELECT id ,dept_name deptName FROM tbl_dept WHERE id = #{id} </select> <resultMap type="com.neuedu.entity.Employee" id="myEmpByStep"> <id column="id" property="id"/> <result column="user_name" property="userName"/> <result column="gender" property="gender"/> <result column="email" property="email"/> <!-- association定义关联对象的封装规则 select:表明当前属性是调用指定的方法查出的结果 column:指定将哪一列的值传给这个方法 流程:使用select指定的方法(传入column指定的这列参数的值)查出对象,并封装给property指定的属性。 --> <association property="depart" select="getDepartById" column="d_id"></association> </resultMap> <!-- public Employee getEmpAndDept(Integer id); --> <select id="getEmpAndDept" resultMap="myEmpByStep"> select * from tbl_employee where id =#{id} </select>
补充:懒加载机制【按需加载,也叫懒加载】:
<!--
在分步查询这里,我们还要讲到延迟加载:
Employee === > Dept:
我们每次查询Employee对象的时候,都将关联的对象查询出来了。
而我们想能不能我在需要部门信息的时候,再去查询,不需要的时候就不用查询了。
答案:可以的
我们只需要在分步查询的基础之上加上两个配置:
1.在mybatis的全局配置文件中加入两个属性:
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
<!-- 开启懒加载机制 ,默认值为true-->
<setting name="lazyLoadingEnabled" value="true"/>
<!-- 开启的话,每个属性都会直接全部加载出来;禁用的话,只会按需加载出来 -->
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
-->
测试:
@Test
public void testGetEmployee(){
EmployeeMapperPlus mapper = openSession.getMapper(EmployeeMapperPlus.class);
Employee employee = mapper.getEmpAndDept(1);
System.out.println(employee.getUserName());
}
此时:可以看到这里只发送了一条SQL语句。
注意:上面我们上面如果关联的是一个对象,我们还可以使用association标签,如果我们关联的是一个集合,
那么该使用谁呢?
======================================映射对多的关联关系==================================================
场景二:查询部门的时候,将部门对应的所有员工信息也查询出来,注释在DepartmentMapper.xml中
第一种:1.修改Department实体类【添加Employee集合,并为该集合提供getter/setter方法】
public class Department { private Integer id; private String deptName; private List<Employee> list; public List<Employee> getList() { return list; } public void setList(List<Employee> list) { this.list = list; } ...... } 建立DepartmentMapper接口文件,并添加如下方法: public Department getDeptByIdPlus(Integer id);
2.sql映射文件中的内容为:【collection:嵌套结果集的方式:使用collection标签定义关联的集合类型元素的封装规则】
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
<!-- public Department getDeptByIdPlus(Integer id); --> <resultMap type="com.neuedu.entity.Department" id="getDeptByIdPlusMap"> <id column="did" property="id"/> <result column="dept_name" property="deptName"/> <!-- collection:定义关联集合类型的属性的封装规则 ofType:指定集合里面元素的类型 --> <collection property="list" ofType="com.neuedu.entity.Employee"> <!-- 定义这个集合中元素的封装规则 --> <id column="eid" property="id"/> <result column="user_name" property="userName"/> <result column="email" property="email"/> <result column="gender" property="gender"/> </collection> </resultMap> <select id="getDeptByIdPlus" resultMap="getDeptByIdPlusMap"> SELECT d.`id` did, d.`dept_name` dept_name,e.`id` eid,e.`user_name` user_name,e.`email` email,e.`gender` gender FROM `tbl_dept` d LEFT JOIN tbl_employee e ON e.`d_id` = d.`id` WHERE d.`id` = #{id} </select>
3.测试方法为:
@Test
public void testGetEmployee(){
DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.class);
Department department = mapper.getDeptByIdPlus(2);
System.out.println(department);
}
第二种:使用分步查询结果集的方式:
1.如果使用分步查询的话,我们的sql语句就应该为:
SELECT * FROM `tbl_dept` WHERE id = 2;
SELECT * FROM `tbl_employee` WHERE d_id = 2;
2.在DepartmentMapper接口文件中添加方法,如下所示:
public Department getDeptWithStep(Integer id);
3.再从EmployeeMapper接口中添加一个方法,如下所示:
public List<Employee> getEmployeeByDeptId(Integer deptId);
并在响应的sql映射文件中添加相应的sql语句
<select id="getEmployeeByDeptId" resultType="com.neuedu.entity.Employee"> select * from tbl_employee where d_id = #{departId} </select>
4.在DepartmentMapper映射文件中:
<resultMap type="com.neuedu.entity.Department" id="getDeptWithStepMap"> <id column="id" property="id"/> <result column="dept_name" property="deptName"/> <collection property="list" select="com.neuedu.mapper.EmployeeMapper.getEmployeeByDeptId" column="id"></collection> </resultMap> <select id="getDeptWithStep" resultMap="getDeptWithStepMap"> SELECT id ,dept_name FROM tbl_dept WHERE id = #{id} </select>
5.测试类:
@Test
public void testGetEmployee(){
DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.class);
Department department = mapper.getDeptWithStep(2);
System.out.println(department);
}
1.映射(一)对多、(多)对多的关联关系=======》【映射"对多"的关联关系】
1.必须使用collection节点进行映射
2.基本示例:
注意:1). ofType指定集合中的元素类型
2).
<!--
collection标签
映射多的一端的关联关系,使用ofType指定集合中的元素类型
columnprefix:指定列的前缀
使用情境:若关联的数据表和之前的数据表有相同的列名,此时就需要给关联的列其"别名".
若有多个列需要起别名,可以为所有关联的数据表的列都加上相同的前缀,然后再映射时指定前缀。
-->