MyBatis基于注解的一对一、一对多和多对多的关系

秒速五厘米 2022-04-17 01:39 265阅读 0赞

**“今年春尽，杨花似雪，犹不见还家”**

## 前言 ##

之前说了[MyBatis常用注解][MyBatis]  
和基于注解的简单增删改查操作。  
现在来了解下MyBatis基于注解的一对一、一对多和多对多的关系。

## 示例 ##

*  一对一：一个人只有一个身份证号

**PersonMapper .java**

import org.apache.ibatis.annotations.*;
    import org.apache.ibatis.mapping.FetchType;
    import org.arunner.domain.Person;
    
    /**
     * @author arunner
     * @date 2018/11/10
     */
    public interface PersonMapper {
    
        @Select("select * from tb_person where id= #{id}")
        @Results({
                @Result(id=true,column = "id",property = "id"),
                @Result(column = "name",property = "name"),
                @Result(column = "age",property = "age"),
                @Result(column = "sex",property = "sex"),
                @Result(column = "card_id",property = "card",one = @One(select = "org.arunner.mapper.CardMapper.selectCardById",fetchType = FetchType.EAGER)),
        })
        Person selectPersonById(Integer id);
    
    }

**CardMapper .java**

import org.apache.ibatis.annotations.One;
    import org.apache.ibatis.annotations.Result;
    import org.apache.ibatis.annotations.Results;
    import org.apache.ibatis.annotations.Select;
    import org.apache.ibatis.mapping.FetchType;
    import org.arunner.domain.Card;
    import org.arunner.domain.Person;
    
    /**
     * @author arunner
     * @date 2018/11/10
     */
    public interface CardMapper {
        @Select("select * from tb_card where id= #{id}")
        Card selectCardById(Integer id);
    }

**测试**

public class OneToOneTest {
        public static void main(String[] args) {
            SqlSession sqlSession = FKSqlSessionFactory.getSqlSession();
            PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);
            Person person = mapper.selectPersonById(1);
            System.out.println(person);
            //查看关联的Card对象
            System.out.println(person.getCard());
            //提交事务
            sqlSession.commit();
            //关闭sqlSession对象
            sqlSession.close();
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0FfUnVubmVy_size_16_color_FFFFFF_t_70]

*  一对多：一个班级对应多个学生

**ClazzMapper .java**

public interface ClazzMapper {
    
        @Select("select * from tb_clazz where id= #{id}")
        @Results({
                @Result(id = true,column = "id",property = "id"),
                @Result(column = "name",property = "name"),
                @Result(column = "code",property = "code"),
                 //@many注解的select属性表示需要关联执行的SQL语句
                 //FetchType.LAZY表示查询的类型是延迟加载
                @Result(column = "id",property = "students",many = @Many(select = "org.fkit.mapper.StudentMapper.selectByClazzId",fetchType = FetchType.LAZY)),
        })
        //根据ID查询班级信息
        Clazz selectById(Integer id);
    }

**StudentMapper .java**

public interface StudentMapper {
    
        @Select("select * from tb_student where clazz_id= #{id}")
        @Results({
                @Result(id=true,column = "id",property = "id"),
                @Result(column = "name",property = "name"),
                @Result(column = "age",property = "age"),
                @Result(column = "sex",property = "sex")
        })
        List<Student> selectByClazzId(Integer id);
    }

**测试**

public class OneToManyTest {
        public static void main(String[] args) {
            SqlSession sqlSession = FKSqlSessionFactory.getSqlSession();
            ClazzMapper mapper = sqlSession.getMapper(ClazzMapper.class);
           //根据ID查询Clazz对象
            Clazz clazz = mapper.selectById(1);
            System.out.println(clazz.getId() + "-" + clazz.getCode() + "-" + clazz.getName());
            //查看关联的学生集合，因为配置延迟加载，所以当使用时才会执行SQL语句
            List<Student> students = clazz.getStudents();
            for (Student student : students) {
                System.out.println(student);
            }
            //提交事务
            sqlSession.commit();
            //关闭sqlSession对象
            sqlSession.close();
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0FfUnVubmVy_size_16_color_FFFFFF_t_70 1]

*  一对多：一个订单对应多个商品，一个商品也可以属于多个订单

**ArticleMapper .java**

public interface ArticleMapper {
    
    	@Select("SELECT * FROM tb_article WHERE id IN (SELECT article_id FROM tb_item WHERE order_id = #{id} ) ")
    	List<Article> selectByOrderId(Integer order_id);
    	
    }

**OrderMapper .java**

public interface OrderMapper {
    
    	@Select("SELECT * FROM TB_ORDER WHERE ID = #{id}")
    	@Results({
    		@Result(id=true,column="id",property="id"),
    		@Result(column="code",property="code"),
    		@Result(column="total",property="total"),
    		@Result(column="user_id",property="user",
    			one=@One(select="org.fkit.mapper.UserMapper.selectById",
    		fetchType=FetchType.EAGER)),
    		@Result(column="id",property="articles",
    			many=@Many(select="org.fkit.mapper.ArticleMapper.selectByOrderId",
    		fetchType=FetchType.LAZY))
    	})
    	Order selectById(Integer id);
    	
    }

**UserMapper .java**

public interface UserMapper {
    
    	@Select("SELECT * FROM TB_USER WHERE ID = #{id} ")
    	User selectById(Integer id);
    	
    }

**测试**

public class ManyToManyTest {
    
    	public static void main(String[] args) throws Exception {
    		// 获取Session实例
    		SqlSession session = FKSqlSessionFactory.getSqlSession();
    		// 获取OrderMapper实例
    		OrderMapper om = session.getMapper(OrderMapper.class);
    		// 根据id查询Order对象
    		Order order = om.selectById(1);
    		// 查看查询到的Order对象
    		System.out.println(order.getId() + " " + order.getCode() + " " + order.getTotal());
    		// 查看Order关联的User对象
    		System.out.println(order.getUser());
    		// 查看关联的Article集合，因为配置使用的是LAZY懒加载，所以当使用时才执行SQL语句
    		order.getArticles().forEach(article -> System.out.println(article));
    		// 提交事务
    		session.commit();
    		// 关闭Session
    		session.close();
    	}
    }

[MyBatis]: https://blog.csdn.net/A_Runner/article/details/83956233
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