Mybatis——关联映射查询

一、一对一映射：

主Model：

 1 package com.neuedu.model;
 2 
 3 import java.util.Date;
 4 
 5 /**
 6  * orders订单表
 7  */
 8 public class Order {
 9     
10     private int id;
11     
12     private int userid;
13     
14     private String orderId;
15     
16     private Date createTime;
17     
18     private String note;
19     
20     //一对一关系
21     private User user;
22 
23     public int getId() {
24         return id;
25     }
26 
27     public void setId(int id) {
28         this.id = id;
29     }
30 
31     public int getUserid() {
32         return userid;
33     }
34 
35     public void setUserid(int userid) {
36         this.userid = userid;
37     }
38 
39     public String getOrderId() {
40         return orderId;
41     }
42 
43     public void setOrderId(String orderId) {
44         this.orderId = orderId;
45     }
46 
47     public Date getCreateTime() {
48         return createTime;
49     }
50 
51     public void setCreateTime(Date createTime) {
52         this.createTime = createTime;
53     }
54 
55     public String getNote() {
56         return note;
57     }
58 
59     public void setNote(String note) {
60         this.note = note;
61     }
62     
63     public User getUser() {
64         return user;
65     }
66 
67     public void setUser(User user) {
68         this.user = user;
69     }
70 
71     @Override
72     public String toString() {
73         return "Order [id=" + id + ", userid=" + userid + ", orderId=" + orderId + ", createTime=" + createTime
74                 + ", note=" + note + ", user=" + user + "]";
75     }
76     
77 }

方式一：通过自定义类实现

（1）Model包中，新建一个类使之继承主查询表Model,添加新对应表Model中的属性

 1 package com.neuedu.model;
 2 /**
 3  * 自定义
 4  */
 5 public class OrderCustomer extends Order{
 6 
 7     private int userid;
 8     
 9     private String username;
10     
11     private String address;
12 
13     public int getUserid() {
14         return userid;
15     }
16 
17     public void setUserid(int userid) {
18         this.userid = userid;
19     }
20 
21     public String getUsername() {
22         return username;
23     }
24 
25     public void setUsername(String username) {
26         this.username = username;
27     }
28 
29     public String getAddress() {
30         return address;
31     }
32 
33     public void setAddress(String address) {
34         this.address = address;
35     }
36 
37     @Override
38     public String toString() {
39         return "OrderCustomer [userid=" + userid + ", username=" + username + ", address=" + address + ", getId()="
40                 + getId() + ", getOrderId()=" + getOrderId() + ", getCreateTime()=" + getCreateTime() + ", getNote()="
41                 + getNote() + "]";
42     }
43     
44 }

（2）mapper.xml中，通过ResultType指向新建的类，进行注入

1 <!-- 一对一查询，需多自定义一个类OrderCustomer -->
2     <select id="selectOrder" resultType="com.neuedu.model.OrderCustomer">
3         select 
4             o.id, o.userid, o.orderid, o.createtime, o.note,
5             t.id userid,t.username,t.birthday,t.address 
6         from orders o,t_user t 
7         where o.userid = t.id
8     </select>

方式二：通过ResultMap方式注入

（1）在mapper.xml中建立ResultMap映射，运用association标签添加一对一关系映射

 1 <!-- 一对一查询，返回resultMap -->
 2     <resultMap id="selectOrderResultMap" type="com.neuedu.model.Order" >
 3         <id property="id" column="id"/>    <!-- property pojo对象的属性名称    column查询语句的列名称-->
 4         <result property="userid" column="userid"/>
 5         <result property="orderId" column="orderid"/>
 6         <result property="createTime" column="createtime"/>
 7         <result property="note" column="note"/>
 8         <!-- 一对一 关系-->
 9         <association property="user" javaType="com.neuedu.model.User">
10             <id property="id" column="userid"/>
11             <result property="username" column="username"/>
12             <result property="birthday" column="birthday"/>
13             <result property="address" column="address"/>
14         </association>
15     </resultMap>

（2）在mapper.xml中编写查询语句，使ResultMap属性指向前面定义的ResultMap的id

1 <select id="selectOrderByResultMap" resultMap="selectOrderResultMap">
2         select 
3             o.id, o.userid, o.orderid, o.createtime, o.note,
4             t.id userid,t.username,t.birthday,t.address 
5         from orders o,t_user t 
6         where o.userid = t.id
7     </select>

二、一对多查询：

主Model中添加属性

1 //一对多关系
2     private List<OrderDetail> orderDetailList;

（一个订单对应多个商品明细）

（1）通过属性collection添加一对多映射

1 <!-- 一对多关系：collection -->
2         <collection property="orderDetailList" ofType="com.neuedu.model.OrderDetail">
3             <id property="id" column="odid"/>
4             <result property="ordersid" column="ordersid"/>
5             <result property="itemsid" column="itemsid"/>
6             <result property="itemsnum" column="itemsnum"/>
7         </collection>

注意：resultMap标签中的extends可指定继承resultMap的标签，可使用其中的所有映射关系，并添加自己独自的