【10】MyBatis 映射关系

1. 关系模型分析

1:1	订单->用户；订单明细->商品；订单明细->订单
1:n	用户->订单；订单->订单明细；
n:m	订单<->商品；用户<->商品

2. 一对一

2.1 需求

• 根据订单Id获取订单信息，包括订单所属的用户信息

2.2 嵌套 pojo 方式

• 实体类
  Orders类属于Mybatis框架中对应数据表Orders的一个基础类，通常由逆向工程生成，每次逆向工程都会覆盖原来的Orders类，为了避免Orders类中的自定义内容被逆向工程覆盖，因此推荐做法：定义Orders的扩展类OrdersExt1

  	public class User {
  
  		private Integer id;
  		private String username;
  		private Date birthday;
  		private String sex;
  		private String address;
  	}
  	
  	public class Order {
  	
  		private int id;
  		private int user_id;
  		private String number;
  		private Date createtime;
  		private String note;
  	}
  	
  	public class OrderExt extends Order {	
  		//多对一
  		private User user;		
  		public User getUser() {
  			return user;
  		}
  		public void setUser(User user) {
  			this.user = user;
  		}
  	}
  

• OrdersMapper接口

  	/**
  	 * 根据编号查询订订单
  	 * @param id
  	 * @return
  	 */
  	public OrderExt findOrderExtById(Integer id);
  

• OrdersMapper.xml

    	
    		
    		
    		
    		
    		
    		
    		
    		
    			
    			
    			
    			
    			
    		
    		
    	
    	
    		SELECT 
    			o.id,
    			o.user_id,
    			o.number,
    			o.createtime,
    			o.note,
    			u.id uid,
    			u.username,
    			u.birthday,
    			u.sex,
    			u.address 
    			FROM `order` o , `user` u 
    			WHERE o.user_id = u.id
    			and o.id = #{id}
    	
  

• 测试

      @Before
    	public void before() throws IOException {
    		// MyBatis 配置文件路径
    		String resource = "mybatis-config.xml";
    		// 通过Resource 读取配置文件输入流
    		InputStream inputStream = Resources.getResourceAsStream(resource);
    		// 创建SQL会话工厂
    		sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    	}
    
    	@Test
    	public void method01() throws IOException {
    
    		SqlSession sqlSession = sqlSessionFactory.openSession();
    		
    		OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
    		OrderExt orderExt = orderMapper.findOrderExtById(1);
    		System.out.println(orderExt);
    		
    		sqlSession.close();
    	}
  

2.3 平铺 pojo 方式

• 实体类

    	public class Order {
    		
    			private int id;
    			private int user_id;
    			private String number;
    			private Date createtime;
    			private String note;
    	}
    		
    	public class OrderExt2 extends Order {	
    		private Integer uid;
    		private String username;
    		private Date birthday;
    		private String sex;
    		private String address;
    	}
  

• 接口方法

    	/**
    	 * 根据编号查询订订单
    	 * @param id
    	 * @return
    	 */
    	public OrderExt2 findOrderExtById(Integer id);
  

• 映射sql

    	
    		SELECT 
    			o.id,
    			o.user_id,
    			o.number,
    			o.createtime,
    			o.note,
    			u.id uid,
    			u.username,
    			u.birthday,
    			u.sex,
    			u.address 
    		FROM `order` o , `user` u 
    		WHERE o.user_id = u.id and o.id = #{id}
    	
  

• 测试

      @Test
      public void method01() throws IOException {
    	    SqlSession sqlSession = sqlSessionFactory.openSession();
    	    		
    	    OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
    	  	OrderExt2 orderExt = orderMapper.findOrderExtById(1);
    	    System.out.println(orderExt);
    	    sqlSession.close();
    	}
  

2.4 两种实现方式比较：

• 平铺式实现方式简单，但是结构不清晰
• 嵌套是实现方式相对复杂（手工映射结果集），结构清晰
• 一般对于结构简单的1:1关系，推荐使用平铺式；对于结果相对负责的1:1关系，推荐使用嵌套式

3. 一对多

3.1 需求

• 根据订单Id查找订单信息（包含用户信息、订单明细）

3.2 开发步骤

1. 步骤1：创建PO类 Items、OrderDetail、OrderDetailExt

• Items类

  	public class Item {
  		private Integer id;
  		private String name;
  		private Double price;
  		private String detail;
  		private String pic;
  		private Date createtime;
  	}
  

• OrderDetail类

  	public class OrderDetail {
  		private Integer id;
  		private Integer orders_id;
  		private Integer items_id;
  		private Integer items_num;
  	}
  	
  

• OrderDetailExt类

  	public class OrderDetailExt extends OrderDetail {
  		private Item item;
  		public Item getItem() {
  			return item;
  		}
  		public void setItem(Item item) {
  			this.item = item;
  		}
  	}
  

2. 步骤2：修改OrdersExt1类关联订单明细信息

   	public class OrderExt extends Order {
   		// 多对一
   		private User user;
   		// 多对一
   		private List orderDetails;
   	
   		public User getUser() {
   			return user;
   		}
   		public void setUser(User user) {
   			this.user = user;
   		}
   		public List getOrderDetails() {
   			return orderDetails;
   		}
   		public void setOrderDetails(List orderDetails) {
   			this.orderDetails = orderDetails;
   		}
   	}
   

3. 步骤3：修改OrdersMapper接口

   	/**
   	 * 根据编号查询订单和详单信息
   	 * @param id
   	 * @return
   	 */
   	public OrderExt findOrderWithDetail(Integer id);
   

4. 步骤4：添加sql映射

   	
   		SELECT
   			o.id,
   			o.user_id,
   			o.number,
   			o.createtime,
   			o.note,
   			od.id orderdetail_id,
   			od.orders_id,
   			od.items_id,
   			od.items_num,
   			u.id uid,
   			u.username,
   			u.birthday,
   			u.sex,
   			u.address
   		FROM `order` o ,`orderdetail` od, `user` u
   		WHERE o.id = od.orders_id
   		AND o.user_id = u.id
   		AND o.id = #{id}
   	
   

5. 步骤5：测试

   	@Test
   	public void method02() throws IOException {
   		SqlSession sqlSession = sqlSessionFactory.openSession();
   		OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
   		OrderExt orderExt = orderMapper.findOrderWithDetail(2);
   		System.out.println(orderExt);
   		sqlSession.close();
   	}
   

4. 多对多

4.1 需求

• 获取所有订单（含有所属用户、订单明细、商品）

4.2 开发步骤

1. 步骤1：添加接口方法

   	/**
   	 * 根据编号查询订单、详单、商品的信息
   	 * @param id
   	 * @return
   	 */
   	public OrderExt findOrderWithDetailAndItem(Integer id);
   

2. 步骤2：添加sql映射

   	
   		SELECT
   			o.id,
   			o.user_id,
   			o.number,
   			o.createtime,
   			o.note,
   			od.id orderdetail_id,
   			od.orders_id,
   			od.items_id,
   			od.items_num,
   			u.id uid,
   			u.username,
   			u.birthday,
   			u.sex,
   			u.address,
   			i.id item_id,
   			i.name,
   			i.price,
   			i.detail,
   			i.pic,
   			i.createtime
   		FROM `order` o ,`orderdetail` od, `user` u , `items` i	
   		WHERE o.id = od.orders_id
   		AND o.user_id = u.id
   		AND o.id = #{id}
   	
   

3. 步骤3：测试

   	@Test
   	public void method03() throws IOException {
   		SqlSession sqlSession = sqlSessionFactory.openSession();
   		OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
   		OrderExt orderExt = orderMapper.findOrderWithDetailAndItem(2);
   		System.out.println(orderExt);
   		sqlSession.close();
   	}