Mybatis - 表關聯one-to-many

one to one - association ; //單對一，使用association one to many -
collection ; //單對多，使用collection

Nested results - column is not necessary ，javaType is necessary ！
//使用嵌套結果，column 不是必需的，可是JavaType是必需的；

nested queries - column is necessary ，javaType is not necessary ！
//使用嵌套查詢，column是必需的，JavaType不是必須的，子查詢自定義resultType便可！！

1. 表結構：

   t_student 表擁有屬性 class_id 對應 t_class表 t_id

【1】更改Classes，添加屬性

public class Classes {

    private int id;
    private String name;
    private Teacher teacher;
    private List<Student> list;
    ...
}

【2】嵌套結果-獲取Classes，Teacher AND list

<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"/>

        <!-- here column is not necessary !!!-->
        <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>

        <!-- here, no column !!! -->
        <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】嵌套查詢–獲取Classes，Teacher AND list

<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 !!! 有屬性select = getTeacher-->
        <association property="teacher" column="t_id" javaType="Teacher" select="getTeacher">
        </association>

        <!-- here,column is necessary !!! 有屬性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】Test

1. 獲取SqlSessionFactory的工具類：

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;
        }

1. 測試方法

   @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=計算機, 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]]