Mybaits基础：多对一

                                         多个产品对应同一个类

参考：http://how2j.cn/k/mybatis/mybatis-many-to-one/1090.html

表结构：

• 产品表

create table product_(
id int NOT NULL AUTO_INCREMENT,
name varchar(30)  DEFAULT NULL,
price float  DEFAULT 0,
cid int ,
PRIMARY KEY (id)
)AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

• 分类表

CREATE TABLE category_ (
  id int(11) NOT NULL AUTO_INCREMENT,
  name varchar(32) DEFAULT NULL,
  PRIMARY KEY (id)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

实体类：

• Product

public class Product {
    private int id;
    private String name;
    private float price;
    private Category category;  //新增类别属性
     
    public Category getCategory() {
        return category;
    }
    public void setCategory(Category category) {
        this.category = category;
    }
    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 float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
    @Override
    public String toString() {
        return "Product [id=" + id + ", name=" + name + ", price=" + price + "]";
    }
 
}

• Category

import java.util.List;
 
public class Category {
    private int id;
    private String name;
    List<Product> products; 
    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<Product> getProducts() {
        return products;
    }
    public void setProducts(List<Product> products) {
        this.products = products;
    }
    @Override
    public String toString() {
        return "Category [id=" + id + ", name=" + name + "]";
    }
     
}

映射文件：

• Product.xml

<!--
通过resultMap ，进行字段和属性的对应。
使用association 进行多对一关系关联，指定表字段名称与对象属性名称的一一对应关系
注： Category的id 字段 和Product的id字段同名，Mybatis不知道谁是谁的，所以需要通过取别名cid,pid来区分。
name字段同理。 
-->

<?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.java.pojo">
        <resultMap type="Product" id="productBean">
            <id column="pid" property="id" />
            <result column="pname" property="name" />
            <result column="price" property="price" />
     
            <!-- 多对一的关系 -->
            <!-- property: 指的是属性名称, javaType：指的是属性的类型 -->
            <association property="category" javaType="Category">
                <id column="cid" property="id"/>
                <result column="cname" property="name"/>
            </association>
        </resultMap>
     
        <!-- 根据id查询Product, 关联将Orders查询出来 -->
        <select id="listProduct" resultMap="productBean">
            select c.*, p.*, c.id 'cid', p.id 'pid', c.name 'cname',
               p.name 'pname' from category_ c left join product_ p on c.id = p.cid
        </select>   
    </mapper>

• mybaits.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>
      <package name="com.java.pojo"/>
    </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/java?characterEncoding=UTF-8"/>
            <property name="username" value="root"/>
            <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/java/pojo/Category.xml"/>
        <mapper resource="com/java/pojo/Product.xml"/>  <!--在mybatis.xml中增加对于Product.xml的映射-->
    </mappers>
</configuration>

运行：

public static void main(String[] args) throws IOException {
        String resource = "mybatis.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
  
        List<Product> ps = session.selectList("listProduct");
        for (Product p : ps) {
            System.out.println(p+" 对应的分类是 \t "+ p.getCategory());
        }
 
        session.commit();
        session.close();
  
    }

输出：