mybatis的3种关联映射（2）：一对多

Dear 丶 2022-05-24 02:08 216阅读 0赞

### mybatis的3种关联映射有：一对一，一对多，多对多。 ###

### 在实际项目开发中经常遇到一对多的关联，比如老师和学生是一对多的关系。 ###

在mapper.xml中写法：

<mapper namespace="springboot.mapper.TeacherMapper" >
      <resultMap id="BaseResultMap" type="springboot.model.Teacher" >
        <id column="tid" property="tid" jdbcType="INTEGER" />
        <result column="tname" property="tname" jdbcType="VARCHAR" />
        <result column="tphone" property="tphone" jdbcType="VARCHAR" />
        <result column="taddr" property="taddr" jdbcType="VARCHAR" />
        <result column="cid" property="cid" jdbcType="INTEGER" />
    
    
          
          <collection property="user" column="tid"  resultMap="springboot.mapper.UserMapper.BaseResultMap" ></collection>
    
    
      </resultMap>
      <sql id="Base_Column_List" >
        tid, tname, tphone, taddr, cid
      </sql>
    
      
       
      <select id="findUser" resultMap="BaseResultMap" parameterType="java.lang.Integer">
        SELECT t.* ,u.*
        FROM teacher t,user u
        WHERE t.tid=u.tid and t.tid = #{tid,jdbcType=INTEGER}
      </select>

dao层写法：

public interface TeacherMapper {
       
        //老师跟学生一对多关系
        Teacher findUser(Integer id);
    
    }

#### 注：在dao层的方法名findUser要跟mapper.xml中的select id中的值保持一致，不一致会报错 ####

model层：

public class Teacher {
        private Integer tid;
    
        private String tname;
    
        private String tphone;
    
        private String taddr;
    
        private Integer cid;
    
    //新增学生列表，一个老师教多个学生
        private List<User> user;
    
        public List<User> getUser() {
            return user;
        }
    
        public void setUser(List<User> user) {
            this.user = user;
        }
    
        public Integer getTid() {
            return tid;
        }
    
        public void setTid(Integer tid) {
            this.tid = tid;
        }
    
        public String getTname() {
            return tname;
        }
    
        public void setTname(String tname) {
            this.tname = tname;
        }
    
        public String getTphone() {
            return tphone;
        }
    
        public void setTphone(String tphone) {
            this.tphone = tphone;
        }
    
        public String getTaddr() {
            return taddr;
        }
    
        public void setTaddr(String taddr) {
            this.taddr = taddr;
        }
    
        public Integer getCid() {
            return cid;
        }
    
        public void setCid(Integer cid) {
            this.cid = cid;
        }
    
        @Override
        public String toString() {
            return "Teacher{" +
                    "tid=" + tid +
                    ", tname='" + tname + '\'' +
                    ", tphone='" + tphone + '\'' +
                    ", taddr='" + taddr + '\'' +
                    ", cid=" + cid +
                    '}';
        }
    }

springboot测试类：

@SpringBootApplication
    @MapperScan("springboot.mapper")
    public class Application
    {
        public static void main( String[] args )
        {
            ApplicationContext context  = SpringApplication.run(Application.class, args);
            TeacherMapper teacherMapper=context.getBean(TeacherMapper.class);
    
            System.out.println("老师和学生一对多联系：");
    
            Teacher teacher2=teacherMapper.findUser(1);
            System.out.println("teacher:"+teacher2.toString());
            List<User>list=teacher2.getUser();
            for (User user:list){
                System.out.println("user:"+user.toString());
            }
    
    
    }

测试效果：

老师和学生一对多联系：
            DEBUG [main] - Creating a new SqlSession
            DEBUG [main] - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7e38a7fe] was not registered for synchronization because synchronization is not active
            DEBUG [main] - Fetching JDBC Connection from DataSource
            DEBUG [main] - JDBC Connection [ProxyConnection[PooledConnection[com.mysql.jdbc.JDBC4Connection@78a287ed]]] will not be managed by Spring
            DEBUG [main] - ==>  Preparing: SELECT t.* ,u.* FROM teacher t,user u WHERE t.tid=u.tid and t.tid = ?
            DEBUG [main] - ==> Parameters: 1(Integer)
            DEBUG [main] - <==      Total: 4
            DEBUG [main] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7e38a7fe]
            DEBUG [main] - Returning JDBC Connection to DataSource
            teacher:Teacher{tid=1, tname='老张', tphone='1234', taddr='广州', cid=1}
            user:User{id=1, uname='小黑', upwd='123', unum='123', sex=0, age=18, addr='广东', tid=1}
            user:User{id=2, uname='小白', upwd='234', unum='234', sex=1, age=21, addr='广州', tid=1}
            user:User{id=6, uname='桌号', upwd='123', unum='463', sex=1, age=12, addr='null', tid=1}
            user:User{id=18, uname='黑杰克', upwd='123', unum='123', sex=1, age=18, addr='进一步', tid=1}

### 注：在实际开发中，一对多关系通常映射为集合对象，而由于多方的数据量可能很大，所以通常使用懒加载fetchType="lazy" ###

<collection property="user" column="tid"  resultMap="springboot.mapper.UserMapper.BaseResultMap" fetchType="lazy" ></collection>

### 而多对一只是关联到一个对象，所以通常使用多表连接直接把数据提取出来。 ###

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