MyBatis一對多和多對一
阿新 • • 發佈：2019-01-06
在學習MyBatis3的過程中，文件上面一直在強調一個id的東西！在做這個實驗的時候，也因為沒有理解清楚id含義而導致一對多的“多”中也只有一條資料。id和result的唯一不同是id表示的結果將是當比較物件例項時用到的標識屬性。這幫助來改進整體表現，特別是快取和嵌入結果對映。所以不同資料的id應該唯一區別，不然導致資料結果集只有一條資料。
2.order
三、對映mapper檔案
四。測試類
一、表
二、實體
1.person
package com.kerwin.mybatis.pojo; import java.util.List; public class Person { private int id; private String name; private List<Orders> 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 List<Orders> getOrderList() { return orderList; } public void setOrderList(List<Orders> orderList) { this.orderList = orderList; } @Override public String toString() { return "Person [id=" + id + ", name=" + name + "]"; } public Person() { super(); // TODO Auto-generated constructor stub } public Person(int id, String name, List<Orders> orderList) { super(); this.id = id; this.name = name; this.orderList = orderList; } }
2.order
package com.kerwin.mybatis.pojo; public class Orders { private int id; private double price; private Person person; public Person getPerson() { return person; } public void setPerson(Person person) { this.person = person; } public int getId() { return id; } public void setId(int id) { this.id = id; } public double getPrice() { return price; } public void setPrice(double price) { this.price = price; } @Override public String toString() { return "Orders [id=" + id + ", price=" + price + "]"; } public Orders() { super(); // TODO Auto-generated constructor stub } }
三、對映mapper檔案
1. PersonMapper.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.kerwin.mybatis.pojo.Person"> <resultMap type="com.kerwin.mybatis.pojo.Person" id="personreSultMap"> <id column="p_id" property="id"/> <result column="name" property="name"/> <collection property="orderList" ofType="com.kerwin.mybatis.pojo.Orders" column="pid"> <id column="o_id" property="id"/> <result column="price" property="price"/> </collection> </resultMap> <select id="selectPersonFetchOrder" parameterType="int" resultMap="personreSultMap" > select p.*,o.* from person p,orders o where o.pid=p.p_id and p.p_id=#{id} </select> </mapper>
2. OrdersMapper.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.kerwin.mybatis.pojo.Orders">
<resultMap type="com.kerwin.mybatis.pojo.Orders" id="OrdersResultMap">
<id column="o_id" property="id"/>
<result column="price" property="price"/>
<association property="person" javaType="com.kerwin.mybatis.pojo.Person">
<id column="p_id" property="id"/>
<result column="name" property="name"/>
</association>
</resultMap>
<select id="selectOrdersFetchPerson" resultMap="OrdersResultMap">
select p.*,o.* from person p,orders o where o.pid=p.p_id and o.o_id=#{id}
</select>
</mapper>3.sqlMapConfig.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>
<typeAliases>
<typeAlias type="com.kerwin.mybatis.pojo.Author" alias="Author"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/kerwin/mybatis/pojo/AuthorMapper.xml"/>
<mapper resource="com/kerwin/mybatis/pojo/PostMapper.xml"/>
<mapper resource="com/kerwin/mybatis/pojo/PersonMapper.xml"/>
<mapper resource="com/kerwin/mybatis/pojo/OrdersMapper.xml"/>
</mappers>
</configuration>四。測試類
/**
*
*/
package com.kerwin.mybatis.test;
import java.io.InputStream;
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 org.junit.BeforeClass;
import org.junit.Test;
import com.kerwin.mybatis.pojo.Orders;
import com.kerwin.mybatis.pojo.Person;
/**
* @author Administrator
*
*/
public class PersonAndOrderTest {
private static SqlSessionFactory sessionFactory;
/**
* @throws java.lang.Exception
*/
@BeforeClass
public static void setUpBeforeClass() throws Exception {
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
InputStream inputStream = Resources.getResourceAsStream("sqlMapConfig.xml");
sessionFactory = factoryBuilder.build(inputStream);
}
//一對多，查詢person（一）級聯查詢訂單order（多）
@Test
public void testSelectPersonFetchOrder() throws Exception {
SqlSession session = sessionFactory.openSession();
Person person = session.selectOne("com.kerwin.mybatis.pojo.Person.selectPersonFetchOrder", 1);
System.out.println(person);
System.out.println(person.getOrderList().size());
for(Orders orders : person.getOrderList()){
System.out.println(orders);
}
session.close();
}
//多對一，查詢訂單order（多）級聯查詢person（一）
@Test
public void testSelectOrdersFetchPerson() throws Exception{
SqlSession session = sessionFactory.openSession();
Orders orders = session.selectOne("com.kerwin.mybatis.pojo.Orders.selectOrdersFetchPerson", 1);
System.out.println(orders);
System.out.println(orders.getPerson());
session.close();
}
}
五、測試結果
1.一對多，查詢person（一）級聯查詢訂單order（多）
2.多對一，查詢訂單order（多）級聯查詢person（一）
注意：兩張表中的主鍵id欄位名要唯一，例如不能都寫id，不然的話，在一對多查詢的時候就會出現：級聯出來的訂單項只有一條記錄。我之前就是將兩張表的主鍵id欄位名都寫為id，導致測試結果級聯出來的多一直只有一條資料，具體如下：
