MyBatis框架

4.7.2  association

  • POJO中的属性可能会是一个对象,我们可以使用联合查询,并以级联属性的方式封装对象.使用association标签定义对象的封装规则

public class Department {

         private Integer id ;

         private String departmentName ;

//  省略 get/set方法

}

 

public class Employee {

         private Integer id ;

         private String lastName;

         private String email ;

         private String gender ;

       private Department dept ;

    // 省略 get/set方法

}

 

 

  • 使用级联的方式:

<select id="getEmployeeAndDept" resultMap="myEmpAndDept" >

                   SELECT e.id eid, e.last_name, e.email,e.gender ,d.id did, d.dept_name FROM tbl_employee e , tbl_dept d   WHERE e.d_id = d.id  AND e.id = #{id}

</select>

 

         <resultMap type="com.atguigu.mybatis.beans.Employee" id="myEmpAndDept">

                   <id column="eid" property="id"/>

                   <result column="last_name" property="lastName"/>

                   <result column="email" property="email"/>

                   <result column="gender" property="gender"/>

             <!-- 级联的方式 -->

                   <result column="did" property="dept.id"/>

                    <result column="dept_name" property="dept.departmentName"/>

</resultMap>

 

  • Association‘

<resultMap type="com.atguigu.mybatis.beans.Employee" id="myEmpAndDept">

                   <id column="eid" property="id"/>

                   <result column="last_name" property="lastName"/>

                   <result column="email" property="email"/>

                   <result column="gender" property="gender"/>

                   <association property="dept" javaType="com.atguigu.mybatis.beans.Department">

                            <id column="did" property="id"/>

                            <result column="dept_name" property="departmentName"/>

                   </association>

</resultMap>

 

       4.7.3  association 分步查询

  • 实际的开发中,对于每个实体类都应该有具体的增删改查方法,也就是DAO层, 因此

对于查询员工信息并且将对应的部门信息也查询出来的需求,就可以通过分步的方式

完成查询。

  • 先通过员工的id查询员工信息
  • 再通过查询出来的员工信息中的外键(部门id)查询对应的部门信息.

<select id="getEmployeeAndDeptStep" resultMap="myEmpAndDeptStep">

                    select id, last_name, email,gender,d_id  from tbl_employee where id =#{id}

</select>

 

<resultMap type="com.atguigu.mybatis.beans.Employee" id="myEmpAndDeptStep">

                   <id column="id"  property="id" />

                   <result column="last_name" property="lastName"/>

                   <result column="email" property="email"/>

                   <result column="gender" property="gender"/>

                   <association property="dept"                        select="com.atguigu.mybatis.dao.DepartmentMapper.getDeptById"

                                     column="d_id" fetchType="eager">

                   </association>

</resultMap>

 

       4.7.4  association 分步查询使用延迟加载

  • 在分步查询的基础上,可以使用延迟加载来提升查询的效率,只需要在全局的

Settings中进行如下的配置:

<!-- 开启延迟加载 -->

<setting name="lazyLoadingEnabled" value="true"/>

<!-- 设置加载的数据是按需还是全部 -->

<setting name="aggressiveLazyLoading" value="false"/>