Table of contents
introduction
Compared with the one-to-one introduced earlier, Mybatis] one-to-one cascade query , the one-to-many relationship is the most contacted in actual development.
Application scenario example: the relationship between users and orders is one-to-many, that is, one user can have multiple orders, while one order belongs to only one user.
Create project and database tables
Create a Spring integration Mybatis project [Spring] Spring integration Mybatis case
Create orderstables and usertables
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=49 DEFAULT CHARSET=utf8;
DROP TABLE IF EXISTS `orders`;
CREATE TABLE `orders` (
`id` int(11) NOT NULL,
`userid` int(11) DEFAULT NULL,
`orderinfo` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `userorder` (`userid`),
CONSTRAINT `userorder` FOREIGN KEY (`userid`) REFERENCES `user` (`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 int userid;
private String orderinfo;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getUserid() {
return userid;
}
public void setUserid(int userid) {
this.userid = userid;
}
public String getOrderinfo() {
return orderinfo;
}
public void setOrderinfo(String orderinfo) {
this.orderinfo = orderinfo;
}
@Override
public String toString() {
return "Order{" +
"id=" + id +
", userid=" + userid +
", orderinfo='" + orderinfo + '\'' +
'}';
}
}
User class
public class User {
private int id;
private String name;
private int age;
private List<Order> orderList;
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 int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public List<Order> getOrderList() {
return orderList;
}
public void setOrderList(List<Order> orderList) {
this.orderList = orderList;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +
", orderList=" + orderList +
'}';
}
}
UserMapper
@Repository("userMapper")
@Mapper
public interface UserMapper {
List<User> findUserOrdersById();
}
OrderMapper
@Repository("orderMapper")
@Mapper
public interface OrderMapper {
List<Order> findOrdersByUserId(int id);
}
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="UserMapper.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">
<!--根据用户id查询订单信息-->
<select id="findOrdersByUserId" parameterType="integer" resultType="com.lucas.mybatis.model.Order">
select * from orders where userid=#{id}
</select>
</mapper>
UserMapper.xml
method 1
<?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.UserMapper">
<resultMap id="userAndOrders" type="com.lucas.mybatis.model.User">
<id property="id" column="id"></id>
<result property="name" column="name"></result>
<result property="age" column="age"></result>
<!--ofType表示集合的元素类型，将id传递给Dao接口中的findOrdersByUserId方法-->
<collection property="orderList" ofType="com.lucas.mybatis.model.Order" column="id" select="com.lucas.mybatis.mapper.OrderMapper.findOrdersByUserId"></collection>
</resultMap>
<select id="findUserOrdersById" resultMap="userAndOrders">
select * from user
</select>
</mapper>
Method 2
<?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.UserMapper">
<resultMap id="userAndOrders" type="com.lucas.mybatis.model.User">
<id property="id" column="id"></id>
<result property="name" column="name"></result>
<result property="age" column="age"></result>
<!--ofType表示集合的元素类型-->
<collection property="orderList" ofType="com.lucas.mybatis.model.Order" >
<id property="id" column="id"></id>
<result property="orderinfo" column="orderinfo"></result>
</collection>
</resultMap>
<select id="findUserOrdersById" resultMap="userAndOrders">
select user.*,orders.id,orders.orderinfo from user ,orders where `user`.id = orders.userid
</select>
</mapper>
add test code
public class App {
public static void main(String[] args) {
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
UserMapper userMapper = (UserMapper) applicationContext.getBean("userMapper");
List<User> users = userMapper.findUserOrdersById();
for (User user : users) {
System.out.println(user);
}
}
}
