Mybatis中对象映射关联之collection使用

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]]