@Select("select id xid,code from order_")
    @Results({
    	@Result(property = "id", column = "xid"),
        @Result(property = "orderItems", javaType = List.class, column = "xid", 
                many = @Many(select = "com.how2java.mapper.OrderItemMapper.listByOrder"))
           })   
	/**
	 **注意，cloum 映射的数据库的表中字段，但是在使用的时候，是以查询结果为准，如果查询结果使用了别名，那么我们在使用的时候，就要填入对应的别名。
	 **@Select("select id xid,code from order_")  ,该查询语句里面的字段之别名，如果没有设定字段的别名（例如select * 这样的查询方式），那么就会使用实际的字段名
	 **在之后的语句中，调用了OrderItemMapper.listByOrder方法，该方法需要传递参数，而参数就是这个xid
	 */
public List<Order> list();
}

package com.one.springboothellowrold;

import com.one.springboothellowrold.springboot.mapper.CategoryMapper;
import com.one.springboothellowrold.springboot.mapper.OrderMapper;
import com.one.springboothellowrold.springboot.mapper.ProductMapper;
import com.one.springboothellowrold.springboot.pojo.Category;
import com.one.springboothellowrold.springboot.pojo.Order;
import com.one.springboothellowrold.springboot.pojo.OrderItem;
import com.one.springboothellowrold.springboot.pojo.Product;
import org.hibernate.annotations.Source;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.List;

@SpringBootTest
class SpringbootHellowroldApplicationTests {


    @Autowired(required = false)
    private CategoryMapper categoryMapper;

    @Autowired(required = false)
    private  ProductMapper productMapper ;

    @Autowired(required = false)
    private OrderMapper orderMapper ;



    //mybatis 一对多
    @Test
    void contextLoads() {
        List<Category> cs = categoryMapper.findAll();
        for(Category c : cs){
            System.out.println(c.getName());
            List<Product> ps = c.getProducts();
            for (Product p : ps) {
                System.out.println("\t"+p.getName());
            }
        }
    }

    //一对一
    @Test
    void onetoone(){
        List<Product> ps = productMapper.findAll() ;
        for(Product p :ps){
            System.out.println(p + "\t 对应的分类是：\t" + p.getCategory().getName());
        }
    }

    //多对多 一个订单有很多产品，一个产品可以属于很多订单，通过orderitem 作为中间表

    @Test
    void manytomany(){
        List<Order> os = orderMapper.findAll();
        for(Order o : os){
            System.out.println(o.getCode());
            List<OrderItem> ois = o.getOrderItems();
            if(null !=ois){
                for(OrderItem oi:ois){
                    //System.out.println(oi.getProduct().getCategory()); 拿不到product的分类信息
                    System.out.format("\t%s\t%f\t%d%n", oi.getProduct().getName(),oi.getProduct().getPrice(),oi.getNumber());
                }
            }

        }


    }



}

package com.how2java;
  
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
 
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
 
import com.how2java.mapper.OrderMapper;
import com.how2java.pojo.Order;
import com.how2java.pojo.OrderItem;
import com.how2java.pojo.Product;
  
public class TestMybatis {
  
    public static void main(String[] args) throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
  
        listOrder(session);
 
        session.commit();
        session.close();
  
    }
 
    private static void listOrder(SqlSession session) {
        OrderMapper mapper =session.getMapper(OrderMapper.class);
        List<Order> os = mapper.list();
        for (Order o : os) {
            System.out.println(o.getCode());
            List<OrderItem> ois= o.getOrderItems();
            if(null!=ois){
                for (OrderItem oi : ois) {
                    System.out.format("\t%s\t%f\t%d%n", oi.getProduct().getName(),oi.getProduct().getPrice(),oi.getNumber());
                }              
            }
 
        }
    }
}