Table of contents
introduction
In Mybatis, many-to-many cascading queries are usually converted into two one-to-many cascading queries to implement
application scenarios. For example: the relationship between users and products is many-to-many, and an order can have multiple products. Use An intermediate table (order record table) can convert the many-to-many cascade into two one-to-many relationships
Create project and database tables
Create a Spring integration Mybatis project [Spring] Spring integration Mybatis case
Create orderstables, producttables, and order_producttables
DROP TABLE IF EXISTS `orders`;
CREATE TABLE `orders` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`orderinfo` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `product`;
CREATE TABLE `product` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`price` double(10,2) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `order_product`;
CREATE TABLE `order_product` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`order_id` int(11) DEFAULT NULL,
`product_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `order_id` (`order_id`),
KEY `product_id` (`product_id`),
CONSTRAINT `order_id` FOREIGN KEY (`order_id`) REFERENCES `orders` (`id`),
CONSTRAINT `product_id` FOREIGN KEY (`product_id`) REFERENCES `product` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
The project structure is as follows:
Create entity classes and database operation interfaces
Order class
public class Order {
private int id;
private String orderinfo;
// 多对多中的另一个一对多
private List<Product> products;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getOrderinfo() {
return orderinfo;
}
public void setOrderinfo(String orderinfo) {
this.orderinfo = orderinfo;
}
public List<Product> getProducts() {
return products;
}
public void setProducts(List<Product> products) {
this.products = products;
}
@Override
public String toString() {
return "Order{" +
"id=" + id +
", orderinfo='" + orderinfo + '\'' +
", products=" + products +
'}';
}
}
Product class
public class Product {
private int id;
private String name;
private double price;
// 多对多中的一个一对多
private List<Order> orders;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
public List<Order> getOrders() {
return orders;
}
public void setOrders(List<Order> orders) {
this.orders = orders;
}
@Override
public String toString() {
return "Product{" +
"id=" + id +
", name='" + name + '\'' +
", price=" + price +
", orders=" + orders +
'}';
}
}
OrderMapper class
@Repository("orderMapper")
@Mapper
public interface OrderMapper {
List<Order> findAllOrdersAndProduct();
}
ProductMapper class
@Repository("productMapper")
@Mapper
public interface ProductMapper {
List<Product> findAllProductAndOrders();
}
Create configuration file
mybatis-config.xml
<?xml version = "1.0" encoding = "UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0 //EN"
"http//mybatis.org/dtd/mybatis-3-config.dtd" >
<configuration>
<settings>
<!--在使用MyBatis嵌套查询方式进行关联查询时，使用MyBatis的延迟加载可以在一定程度上提高查询效率-->
<!--打开延迟加载的开关-->
<setting name="lazyLoadingEnabled" value="true"/>
<!--将积极加载改为按需加载-->
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
<mappers>
<mapper resource="ProductMapper.xml"/>
<mapper resource="OrderMapper.xml"/>
</mappers>
</configuration>
OrderMapper.xml
<?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.lucas.mybatis.mapper.OrderMapper">
<resultMap id="orderAndProducts" type="com.lucas.mybatis.model.Order">
<id property="id" column="id"></id>
<result property="orderinfo" column="orderinfo"></result>
<collection property="products" ofType="com.lucas.mybatis.model.Product" >
<id property="id" column="id"></id>
<result property="name" column="name"></result>
<result property="price" column="price"></result>
</collection>
</resultMap>
<select id="findAllOrdersAndProduct" resultMap="orderAndProducts">
SELECT orders.*,product.id,product.`name`,product.price FROM orders,product,order_product WHERE orders.id = order_product.order_id AND order_product.product_id = product.id;
</select>
</mapper>
ProductMapper.xml
<?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.lucas.mybatis.mapper.ProductMapper">
<resultMap id="productAndOrders" type="com.lucas.mybatis.model.Product">
<id property="id" column="id"></id>
<result property="name" column="name"></result>
<result property="price" column="price"></result>
<collection property="orders" ofType="com.lucas.mybatis.model.Order" >
<id property="id" column="id"></id>
<result property="orderinfo" column="orderinfo"></result>
</collection>
</resultMap>
<select id="findAllProductAndOrders" resultMap="productAndOrders">
SELECT product.*,orders.id,orders.orderinfo FROM orders,product,order_product WHERE orders.id = order_product.order_id AND order_product.product_id = product.id;
</select>
</mapper>
add test code
public class App {
public static void main(String[] args) {
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
OrderMapper orderMapper = (OrderMapper) applicationContext.getBean("orderMapper");
System.out.println("----订单列表-----");
List<Order> orders = orderMapper.findAllOrdersAndProduct();
for (Order order : orders) {
System.out.println(order);
}
System.out.println("----商品列表列表-----");
ProductMapper productMapper = (ProductMapper) applicationContext.getBean("productMapper");
List<Product> products = productMapper.findAllProductAndOrders();
for (Product product : products) {
System.out.println(product);
}
}
}
The result of the operation is as follows:
