参考博文：
MyBatis中映射器之结果映射详解
MyBatis中对象映射关联之association使用实践
写在前言
one to one - association ;
//单对一，使用association
one to many - collection ;
//单对多，使用collection
Nested results - column is not necessary ，javaType is necessary ！
//使用嵌套结果，column 不是必需的，但是JavaType是必需的；
nested queries - column is necessary ，javaType is not necessary ！
//使用嵌套查询，column是必需的，JavaType不是必须的，子查询自定义resultType即可！！
表结构：
t_student 表拥有属性 class_id 对应 t_class表 t_id
【1】更改Classes，添加属性
Classes模型中包括多个Student：
public class Classes { private int id; private String name; private Teacher teacher; private List<Student> list; ... }
【2】嵌套结果-获取Classes，Teacher and list
下面实例讲关联都写在了getClass4Map结果映射中，其实association与collection的结果映射可以单独拎出来。
<select id="getClass4" parameterType="int" resultMap="getClass4Map"> select * from t_class c,t_student s ,t_teacher where c.c_id = s.class_id and c.t_id = t_teacher.t_id and c.c_id = #{id} </select> <resultMap type="Classes" id="getClass4Map"> <result property="id" column="c_id" javaType="int" jdbcType="INTEGER"/> <result property="name" column="c_name" javaType="string" jdbcType="VARCHAR"/> <association property="teacher" column="t_id" javaType="Teacher" > <id property="id" column="t_id" javaType="int" jdbcType="INTEGER"/> <result property="name" column="t_name" javaType="string" jdbcType="VARCHAR"/> </association> <collection property="list" ofType="Student" > <result property="id" column="s_id" javaType="int" jdbcType="INTEGER"/> <result property="name" column="s_name" javaType="string" jdbcType="VARCHAR"/> </collection> </resultMap>
【3】嵌套查询–获取Classes，Teacher and list
执行查询getClass5时，将会级联查询getTeacher和getStudent。
<select id="getClass5" parameterType="int" resultMap="getClass5Map"> select * from t_class c where c.c_id = #{id} </select> <select id="getTeacher" parameterType="int" resultType="Teacher"> select t_id id,t_name name from t_teacher where t_id = #{id} </select> <select id="getStudent" parameterType="int" resultType="Student"> select s_id id,s_name name from t_student t where t.class_id = #{id} </select> <resultMap type="Classes" id="getClass5Map"> <result property="id" column="c_id" javaType="int" jdbcType="INTEGER"/> <result property="name" column="c_name" javaType="string" jdbcType="VARCHAR"/> <!-- here,column is necessary !!! 有属性select = getTeacher--> <association property="teacher" column="t_id" javaType="Teacher" select="getTeacher"> </association> <!-- here,column is necessary !!! 有属性select = getStudent --> <collection property="list" ofType="Student" select="getStudent" column="c_id"> </collection> <!-- pay attention to the Query condition of SQL statement t.class_id = #{id} not t.s_id = #{id} !!--> </resultMap>
【4】代码测试
获取SqlSessionFactory的工具类：
public static SqlSessionFactory getFactory(){ /* flow the src dir*/ String resource = "mybatis.xml"; /*MybatisUtils.class.getResourceAsStream(resource)----- it's wrong !!!! * please distinguish the two up and down * */ InputStream inputStream = MybatisUtils.class.getClassLoader().getResourceAsStream(resource); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream); return factory; }
测试方法
@Test public void testSelect4(){ /*set auto commit ,which equals to the above*/ SqlSession session = MybatisUtils.getFactory().openSession(true); String statement = "com.web.mapper.classMapper.getClass4"; /*return the effect rows*/ Classes classes = session.selectOne(statement, 1); Teacher teacher = classes.getTeacher(); List<Student> list = classes.getList(); System.out.println("result.."+classes+','+classes.getClass()); System.out.println(teacher); System.out.println(list); }
result as follows :
result..Classes [id=1, list=[Student [id=1, name=stu1], Student [id=2, name=stu2], Student [id=3, name=stu3]], name=计算机, teacher=Teacher [id=1, name=李明]],class com.web.model.Classes Teacher [id=1, name=李明] [Student [id=1, name=stu1], Student [id=2, name=stu2], Student [id=3, name=stu3]]
