Mybatis 學習筆記（五）——關聯（對映）關係（多對多）
阿新 • • 發佈：2019-02-01
三、多對多對映關係
1. 需求：查詢使用者及商品資訊
2. POJO類
/mybatis01/src/com/po/User.java
package com.po; import java.util.Date; import java.util.List; /** * 使用者類 * @author 歐陽 * */ public class User { private int id; //id private String username; //使用者名稱 private String sex; //性別 private Date birthday; //生日 private String address; //地址 private List<Orders> orderslist; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } public Date getBirthday() { return birthday; } public void setBirthday(Date birthday) { this.birthday = birthday; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } public List<Orders> getOrderslist() { return orderslist; } public void setOrderslist(List<Orders> orderslist) { this.orderslist = orderslist; } }
/mybatis01/src/com/po/Orders.java
package com.po; import java.util.Date; import java.util.List; /** * 訂單類 * @author 歐陽 * */ public class Orders { private Integer id; //id private Integer userId; //使用者id private String number; //數量 private Date createtime; //建立時間 private String note; //備註 private User user; //使用者資訊 private List<Orderdetail> orderdetail; //訂單明細 public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public Integer getUserId() { return userId; } public void setUserId(Integer userId) { this.userId = userId; } public String getNumber() { return number; } public void setNumber(String number) { this.number = number == null ? null : number.trim(); } public Date getCreatetime() { return createtime; } public void setCreatetime(Date createtime) { this.createtime = createtime; } public String getNote() { return note; } public void setNote(String note) { this.note = note == null ? null : note.trim(); } public User getUser() { return user; } public void setUser(User user) { this.user = user; } public List<Orderdetail> getOrderdetail() { return orderdetail; } public void setOrderdetail(List<Orderdetail> orderdetail) { this.orderdetail = orderdetail; } }
/mybatis01/src/com/po/Orderdetail.java
package com.po; /** * 訂單詳情類 * @author 歐陽 * */ public class Orderdetail { private Integer id; private Integer ordersId; private Integer itemsId; private Integer itemsNum; private Items items; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public Integer getOrdersId() { return ordersId; } public void setOrdersId(Integer ordersId) { this.ordersId = ordersId; } public Integer getItemsId() { return itemsId; } public void setItemsId(Integer itemsId) { this.itemsId = itemsId; } public Integer getItemsNum() { return itemsNum; } public void setItemsNum(Integer itemsNum) { this.itemsNum = itemsNum; } public Items getItems() { return items; } public void setItems(Items items) { this.items = items; } }
/mybatis01/src/com/po/Items.java
package com.po;
import java.util.Date;
/**
* 商品類
* @author 歐陽
*
*/
public class Items {
private Integer id;
private String name;
private Float price;
private String pic;
private Date createtime;
private String detail;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name == null ? null : name.trim();
}
public Float getPrice() {
return price;
}
public void setPrice(Float price) {
this.price = price;
}
public String getPic() {
return pic;
}
public void setPic(String pic) {
this.pic = pic == null ? null : pic.trim();
}
public Date getCreatetime() {
return createtime;
}
public void setCreatetime(Date createtime) {
this.createtime = createtime;
}
public String getDetail() {
return detail;
}
public void setDetail(String detail) {
this.detail = detail == null ? null : detail.trim();
}
@Override
public String toString() {
return "Items [id=" + id + ", name=" + name + ", price=" + price
+ ", pic=" + pic + ", createtime=" + createtime + ", detail="
+ detail + "]";
}
}
3. 編寫Mapper.xml
將下面的程式碼加入 OrdersCustomMapper.xml 中
<!--
查詢使用者及使用者購買商品資訊(多對多)
-->
<resultMap type="com.po.User" id="UserAndItemsResultMap">
<!-- 使用者資訊 -->
<id column="u_id" property="id"/>
<result column="username" property="username"/>
<result column="sex" property="sex"/>
<result column="address" property="address"/>
<!-- 配置對映的訂單資訊 -->
<collection property="orderslist" ofType="com.po.Orders">
<id column="id" property="id"/>
<result column="user_id" property="userId"/>
<result column="number" property="number"/>
<result column="createtime" property="createtime"/>
<result column="note" property="note"/>
<!-- 配置對映的訂單明細資訊 -->
<collection property="orderdetail" ofType="com.po.Orderdetail">
<id column="od_id" property="id"/>
<result column="orders_id" property="ordersId"/>
<result column="items_id" property="itemsId"/>
<result column="items_num" property="itemsNum"/>
<!-- 配置對映的商品資訊 -->
<association property="items" javaType="com.po.Items">
<id column="i_id" property="id"/>
<result column="name" property="name"/>
<result column="detail" property="detail"/>
<result column="price" property="price"/>
</association>
</collection>
</collection>
</resultMap>
<select id="findUserAndItemsResultMap" resultMap="UserAndItemsResultMap">
SELECT
o.*,
u.id u_id,
u.username,
u.sex,
u.address,
od.id od_id,
od.items_id,
od.items_num,
od.orders_id,
i.id i_id,
i.name,
i.detail,
i.price
FROM
orders o,
USER u,
orderdetail od,
items i
WHERE
o.user_id = u.id
AND od.orders_id = o.id
AND od.items_id = i.id
</select>
4. 編寫Mapper介面
將下面程式碼新增到 OrdersCustomMapper.java 中
/*
* 查詢使用者及商品資訊
*/
public List<User> findUserAndItemsResultMap() throws Exception;
5. Mapper介面測試
將下面程式碼加入 OrdersCustomMapperTest.java 中
@Test
public void testFindUserAndItemsResultMap() {
SqlSession sqlsession = sqlSessionFactory.openSession();
OrdersCustomMapper mapper = sqlsession
.getMapper(OrdersCustomMapper.class);
try {
List<User> users = mapper.findUserAndItemsResultMap();
System.out.println(users.size());
} catch (Exception e) {
e.printStackTrace();
}
}
四、總結
個人覺得這個對映關係配置非常簡單，只要掌握了規律就可以編寫出。在POJO編寫、Mapper.xml編寫、Mapper介面編寫這三步中，最難的就是POJO編寫。
在知道需求之後，首先第一步要確定查詢的哪張表是主表，哪些是從表，在分析出這個之後，在主表對應的POJO的基礎之上增加從表對應的POJO為屬性（一對一則為屬性，一對多為集合屬性），然後再分析該POJO（屬性）相應的表下是否有從表，接著在該POJO的基礎上再增加從表對應的POJO為屬性（一對一則為屬性，一對多為集合屬性），以此類推直到沒有從表了。
在編寫Mapper.xml檔案時，一般屬性用 result ；一對一用 association ；一對多用 collection；而多對多是用 result 、association 、collection三個之間相互巢狀表示，按照我編寫POJO的思路根據POJO的屬性和主從的先後關係一步一步的編寫 Mapper.xml 檔案就不會出錯。
而在編寫Mapper介面時只需要安裝開發規範編寫即可。