（六）Mybatis的入门教程—— resultMap的应用（关联查询）和collection相关用法

场景：查询部门的时候，把部门对应的所有员工信息也查询出来

javabean

public class Department{
    private Integer id;
    private String departmentName;
    private List<Employee> emps;
//省略setter、getter方法
}

接口

public interface DepartmentMapper{
    public Department getDeptByIdPlus(Integer id);
}

查询演示

映射文件

collection：定义关联集合类型的属性的封装规则（嵌套结果集的方式）

ofType：指定集合里面元素的类型

再在collection标签中定义这个集合中元素的封装规则

id；result；跟association类似。

<resultMap type="com.mybatis.bean.Department" id="MyDept">
    <id column="did" property="id" />
    <result column="dept_name" property="departmentName" />
    <collection property="emps" ofTypr="com.mybatis.bean.Employee">
        <id column="did" property="id" />
        <result column="last_name" property="lastName" />
        <result column="email" property="email" />
        <result column="gender" property="gender" />
    </collection>
    
</resultMap>

<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 tbl_dept d
    left join tbl_employee e on d.id=e.d_id
    where d.id=#{id}
</select>

测试

@Test
public void test () throws IOException{
    sqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
    sqlSession openSession = sqlSessionFactory.openSession();
    try{
        DepartmentMapperPlus mapper = openSession.getMapper(EmployeeMapper.class);
        Department department = mapper.getDepByIdPlus(1);
        System.out.println(department);
        System.out.println(department.getemps());
    }finally{
        openSession.close();
    }
}

结果

collection的分步查询和延迟加载

接口

public interface DepartmentMapper {
    public Department getDeptByIdStep(Integer id);
    public List<Employee> getEmpsByDeptId(Integer id);
}

相应的映射文件

<select id="getEmpsByDeptId" resultType="com.mybatis.bean.Employee">
    select * from tbl_employee where d_id=#{deptId}
</select> 

<resultMap type="com.mybatis.bean.Department" id="MyDeptStep">
    <id column="id" property="id" />
    <id column="dept_name" property="departmentName" />
    <collection property="emps"
        select="com.mybatis.dao.EmployeeMapperPlus.getEmpsByDeptId"
        column="id" fetchType="lazy">
    </collection>
</resultMap>

<select id="getDeptByIdStep" resultMap="MyDeptStep">
    select id,dept_name  from tbl_dept where id=#{id}
</select>

测试

@Test
public void test () throws IOException{
    sqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
    sqlSession openSession = sqlSessionFactory.openSession();
    try{
        DepartmentMapper mapper = openSession.getMapper(EmployeeMapper.class);
        Department deptByIdStep = mapper.deptByIdStep(1);
        System.out.println(deptByIdStep);
        System.out.println(deptByIdStep.getEmps());
    }finally{
        openSession.close();
    }
}

结果

扩展：在association或者collection中药将多列的值传递过去方法

将多列的值封装成map传递；

column="{  key1=column1，key2=column2 }"

在分步查询的时候出现的 fetchType="lazy"；标识使用延迟加载（lazy）；立即加载（eager）

意思是，就算开启了全局延迟加载，也可以在此处定义成立即加载的属性。