Mybatis之collection与association标签

collection与association标签的功能就是为了解决查询条件映射到一个类或一个集合上，适用于对于多对一，一对多的映射结果，现在我们就探究其具体使用吧。

• 数据库搭建

CREATE TABLE teacher (
  id INT(10) NOT NULL,
  name VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (id)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO teacher(id, name) VALUES (1, ‘秦老师‘); 

CREATE TABLE student (
  id INT(10) NOT NULL,
  name VARCHAR(30) DEFAULT NULL,
  tid INT(10) DEFAULT NULL,
  PRIMARY KEY (id),
  KEY fktid (tid),
  CONSTRAINT fktid FOREIGN KEY (tid) REFERENCES teacher (id)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO student (id, name, tid) VALUES (‘1‘, ‘小明‘, ‘1‘);
INSERT INTO student (id, name, tid) VALUES (‘2‘, ‘小红‘, ‘1‘);
INSERT INTO student (id, name, tid) VALUES (‘3‘, ‘小张‘, ‘1‘);
INSERT INTO student (id, name, tid) VALUES (‘4‘, ‘小李‘, ‘1‘);
INSERT INTO student (id, name, tid) VALUES (‘5‘, ‘小王‘, ‘1‘); 

• pojo

//GET,SET,ToString，有参，无参构造
public class Teacher {
    private int id;
    private String name;
}

//Get,Set,ToString, 有参，无参构造
public class Student {
    private int id;
    private String name;
    //多个学生可以是同一个老师，即多对一
    private Teacher teacher;
}

• 多个学生对应一个老师
• 如果对于学生这边，就是一个多对一的现象，即从学生这边关联一个老师！

接口编写方法：

//获取所有学生及对应老师的信息
public List<Student> getStudents();

配置文件：

方式一：按查询嵌套查询

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.ttt.mapper.StudentMapper">

    <select id="getStudents" resultMap="StudentTeacher">
      select * from student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <!--association关联属性  property属性名 javaType属性类型 column在多的一方的表中的列名-->
        <association property="teacher"  column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    <!--
    这里传递过来的id，只有一个属性的时候，下面可以写任何值
    association中column多参数配置：
        column="{key=value,key=value}"
        其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。
    -->
    <select id="getTeacher" resultType="teacher">
        select * from teacher where id = #{id}
    </select>

</mapper>

方式二：按结果嵌套查询

<select id="getStudents2" resultMap="StudentTeacher2" >
    select s.id sid, s.name sname , t.name tname
    from student s,teacher t
    where s.tid = t.id
</select>

<resultMap id="StudentTeacher2" type="Student">
    <id property="id" column="sid"/>
    <result property="name" column="sname"/>
    <!--关联对象property 关联对象在Student实体类中的属性-->
    <association property="teacher" javaType="Teacher">
        <result property="name" column="tname"/>
    </association>
</resultMap>

在核心配置文件注册。

测试：

 1 @Test
 2 public void testGetStudents(){
 3     SqlSession session = MybatisUtils.getSession();
 4     StudentMapper mapper = session.getMapper(StudentMapper.class);
 5
 6     List<Student> students = mapper.getStudents();
 7
 8     for (Student student : students){
 9         System.out.println(
10                 "学生名:"+ student.getName()
11                         +"\t老师:"+student.getTeacher().getName());
12     }
13 }

testGetStudents

• 一个老师拥有多个学生
• 如果对于老师这边，就是一个一对多的现象。

接口编写方法：

//获取指定老师，及老师下的所有学生
public Teacher getTeacher(int id);

mapper配置文件：

方式一：按查询嵌套查询

<select id="getTeacher2" resultMap="TeacherStudent2">
  select * from teacher where id = #{id}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
    <!--column是一对多的外键 , 写的是一的主键的列名-->
    <collection property="students" javaType="ArrayList" ofType="Student" column="id" select="getStudentByTeacherId"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
    select * from student where tid = #{id}
</select>

方式二：按结果嵌套查询

<mapper namespace="com.ttt.mapper.TeacherMapper">
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname , t.name tname, t.id tid
        from student s,teacher t
        where s.tid = t.id and t.id=#{id}
    </select>

    <resultMap id="TeacherStudent" type="Teacher">
        <result  property="name" column="tname"/>
        <collection property="students" ofType="Student">
            <result property="id" column="sid" />
            <result property="name" column="sname" />
            <result property="tid" column="tid" />
        </collection>
    </resultMap>
</mapper>

在核心配置文件注册。

测试：

1 @Test
2 public void testGetTeacher2(){
3     SqlSession session = MybatisUtils.getSession();
4     TeacherMapper mapper = session.getMapper(TeacherMapper.class);
5     Teacher teacher = mapper.getTeacher2(1);
6     System.out.println(teacher.getName());
7     System.out.println(teacher.getStudents());
8 }

testGetTeacher

1、关联-association  集合-collection

2、所以association是用于一对一和多对一，而collection是用于一对多的关系

3、JavaType和ofType都是用来指定对象类型的

• JavaType是用来指定pojo中属性的类型
• ofType指定的是映射到list集合属性中pojo的类型。

原文地址：https://www.cnblogs.com/zitai/p/11830698.html