resultMap2_关联查询collection分步查询&延迟加载

1、DepartmentMapper.java添加代码

public Department getDeptByIdStep(Integer id);

2、EmployeeMapperPlus.java添加代码

public Employee getEmpByIdDeptId(Integer deptId);

3、DepartmentMapper.xml添加代码

<!-- public Department getDeptByIdStep(Integer id); -->
    <resultMap type="com.atguigu.mybatis.bean.Department" id="MyDeptStep">
        <id column="id" property="id"/>
        <result column="departmentName" property="departmentName"/>
        <collection property="emps" 
                            select="com.atguigu.mybatis.dao.EmployeeMapperPlus.getEmpByIdDeptId"
                                column="id">
        </collection>    
    </resultMap>
    <select id="getDeptByIdStep" resultMap="MyDeptStep">
        select id,dept_name departmentName from tbl_dept where id=#{id}
    </select>

注：collection类似于association，先调用getDeptByIdStep方法获得tbl_dept中id的值，在用获取的id值做为参数传入到com.atguigu.mybatis.dao.EmployeeMapperPlus接口中getDeptByIdStep方法得到Employee 的所有参数值，最后封装到Department对象中 

4、EmployeeMapperPlus.xml添加代码

<!-- public Employee getEmpByIdDeptId(Integer deptId); -->
    <resultMap type="com.atguigu.mybatis.bean.Employee" id="emps22">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
    </resultMap>
    <select id="getEmpByIdDeptId" resultMap="emps22">
        select * from tbl_employee where d_id=#{deptId}
    </select>