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。