[mybatis]映射文件_select_resultMap_关联查询_association分步查询&延迟加载

Posted 2022-02-12 唐火

association分步查询

场景一

查询Employee的同时查询员工对应的部门
Employee===Department
一个员工有与之对应的部门信息

Employee表:

Department表：

public interface DepartmentMapper 

    public Department getDeptById(Integer id);
    

public interface EmployeeMapperPlus 

    public Employee getEmpByIdStep(Integer id);
    

    <!--        public Department getDeptById(Integer id);
-->
    <select id="getDeptById" resultType="com.atguigu.mybatis.bean.Department">
        select id,dept_name departmentName from tb1_dept where id = #id

    </select>

 <resultMap id="MyEmpStep" type="com.atguigu.mybatis.bean.Employee">
        <!--        1.先按照员工id查询员工信息-->
        <!--        2.根据查询员工信息中的d_id值去部门表查出部门信息-->
        <!--        3.部门设置到员工中-->

        <id column="id" property="id"></id>
        <result column="last_name" property="lastName"></result>
        <result column="email" property="email"></result>
        <result column="gender" property="gender"></result>
        <!--        association定义关联对象的封装规则
                    select:表明当前属性是调用select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="dept" select="com.atguigu.mybatis.dao.DepartmentMapper.getDeptById"
                     column="d_id"></association>


    </resultMap>



    <!--    public Employee getEmpByIdStep(Integer id);-->
    <select id = "getEmpByIdStep" resultMap="MyEmpStep">
        select * from tb1_employee where id = #id
    </select>

    @Test
    public void test03() throws IOException 
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();

        SqlSession sqlSession = sqlSessionFactory.openSession();

        try
        
            EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);

            Employee emp = mapper.getEmpByIdStep(1);

            System.out.println(emp);
            System.out.println(emp.getDept());

        finally 

            sqlSession.close();

        
    

延迟加载

Employee ===> Dept
我们每次查询Employee对象的时候，都将一起查询出来
分段查询的基础上，使用延迟加载
可以使部门信息在我们使用的时候再去查询

lazyLoadingEnabled & aggressiveLazyLoading

    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

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

public class Department 

    private Integer id;
    private String departmentName;
    private List<Employee> emps;
    

public interface DepartmentMapper 


    public Department getDeptByIdPlus(Integer id);
    

<resultMap id="MyDept" type="com.atguigu.mybatis.bean.Department">
        <id column="did" property="id"></id>

        <result column="dept_name" property="departmentName"></result>
        <!--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="emps" ofType="com.atguigu.mybatis.bean.Employee" >
            <!--定义这个集合中元素的封装规则-->
            <id column="eid" property="id"></id>
            <result column="last_name" property="lastName"></result>
            <result column="email" property="email"></result>
            <result column="gender" property="gender"></result>
        </collection>
    </resultMap>
    <!--    public Department getDeptByIdPlus(Integer id);-->
    <select id="getDeptByIdPlus" resultMap = "MyDept">
    SELECT d.id did, d.dept_name dept_name ,

e.id eid,e.last_name last_name,e.email email,e.gender gender

FROM tb1_dept d
LEFT JOIN tb1_employee e
ON d.id = e.d_id
WHERE d.id = #id

</select>

    @Test
    public void test04() throws IOException 
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();

        SqlSession sqlSession = sqlSessionFactory.openSession();

        try
        
            DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);

            Department deptByIdPlus = mapper.getDeptByIdPlus(1);

            System.out.println(deptByIdPlus);

            System.out.println(deptByIdPlus.getEmps());

        finally 

            sqlSession.close();

        
    

扩展：多列的值传递过去

将多列的值封装map传递
column="key1=column1,key2=column2"
fetchType=“lazy”：表示使用延迟加载；
lazy：延迟
eager：立即