请问一下Ibatis3/Mybatis该如何进行一对一联合查询且不出现N+1问题

我的两个表：CREATE TABLE `t_customers` (
  `customer_id` int(32) NOT NULL auto_increment,
  `customer_name` varchar(50) default NULL,
  `customer_address` varchar(200) default NULL,
  PRIMARY KEY  (`customer_id`)
)
CREATE TABLE `t_orders` (
  `ORDER_ID` int(32) NOT NULL auto_increment,
  `ORDER_NUMBER` varchar(32) default NULL,
  `ORDER_DATE` date default NULL,
  `ORDER_TOTAL` double(10,2) default NULL,
  `PAYMENT_STATE` int(1) default NULL,
  `SEND_STATE` int(1) default NULL,
  `ORDER_CUSTOMER_ID` int(32) default NULL,
  PRIMARY KEY  (`ORDER_ID`),
  KEY `fk_orders_customers` (`ORDER_CUSTOMER_ID`),
  CONSTRAINT `fk_orders_customers` FOREIGN KEY (`ORDER_CUSTOMER_ID`) REFERENCES `t_customers` (`customer_id`)
-- 每一个Order(订单)对应一个Customer(客户)，t_customer的主键是t_orders的一个外键
) Order.java:package com.yuwen.domain;import java.math.BigDecimal;
import java.util.Date;public class Order {

private long orderId;

private String orderNumber;

private Date orderDate;

private BigDecimal orderTotal;

private int paymentState;

private int sendState;

private Customer customer; public Customer getCustomer() {
return customer;
} public void setCustomer(Customer customer) {
this.customer = customer;
} public long getOrderId() {
return orderId;
} public void setOrderId(long orderId) {
this.orderId = orderId;
} public String getOrderNumber() {
return orderNumber;
} public void setOrderNumber(String orderNumber) {
this.orderNumber = orderNumber;
} public Date getOrderDate() {
return orderDate;
} public void setOrderDate(Date orderDate) {
this.orderDate = orderDate;
}
public BigDecimal getOrderTotal() {
return orderTotal;
} public void setOrderTotal(BigDecimal orderTotal) {
this.orderTotal = orderTotal;
} public int getPaymentState() {
return paymentState;
} public void setPaymentState(int paymentState) {
this.paymentState = paymentState;
} public int getSendState() {
return sendState;
} public void setSendState(int sendState) {
this.sendState = sendState;
}

}Customer.java:package com.yuwen.domain;public class Customer { private long customerId;

private String customerName;

private String customerAddress;

public long getCustomerId() {
return customerId;
} public void setCustomerId(long customerId) {
this.customerId = customerId;
} public String getCustomerName() {
return customerName;
} public void setCustomerName(String customerName) {
this.customerName = customerName;
} public String getCustomerAddress() {
return customerAddress;
} public void setCustomerAddress(String customerAddress) {
this.customerAddress = customerAddress;
}
}dao-configuration.xml:<?xml version="1.0" encoding="UTF-8"?>  
  
<!DOCTYPE configuration PUBLIC "-//ibatis.apache.org//DTD Config 3.0//EN"   
"http://ibatis.apache.org/dtd/ibatis-3-config.dtd">  
  
<configuration>  
   <properties resource="properties/config.properties"/>
   <settings>
   <setting name="lazyLoadingEnabled" value="true"/>  
   </settings>
      <typeAliases>  
        <typeAlias type="com.yuwen.domain.Order" alias="order" />  
        <typeAlias type="com.yuwen.domain.Customer" alias="customer"/>
    </typeAliases>  
    <environments default="development">  
        <environment id="development">  
            <transactionManager type="JDBC"/>  
            <dataSource type="POOLED">
                <property name="driver" value="${driver}" />  
                <property name="url" value="${url}" />  
                <property name="username" value="${username}" />  
                <property name="password" value="${password}" />  
            </dataSource>  
        </environment>  
    </environments>  
    <mappers>  
        <mapper resource="com/yuwen/domain/Order.xml" />
    </mappers>  
       
</configuration>  我认为问题就出在这的Order.xml：<?xml version="1.0" encoding="UTF-8"?>  
<!DOCTYPE mapper PUBLIC "-//ibatis.apache.org//DTD Mapper 3.0//EN"  
"http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">  <mapper namespace="com.yuwen.dao.OrderssDao"> <resultMap type="customer" id="customerMapper">
<id property="customerId" column="customer_id" javaType="long" jdbcType="INTEGER"/>
<result property="customerName" column="customer_name" javaType="java.lang.String" jdbcType="VARCHAR"/>
<result property="customerAddress" column="customer_address" javaType="java.lang.String" jdbcType="VARCHAR"/>
</resultMap>
<resultMap type="order" id="orderMapper">
<id property="orderId" column="ORDER_ID" javaType="long" jdbcType="INTEGER"/>
<result column="ORDER_NUMBER" property="orderNumber" javaType="java.lang.String" jdbcType="VARCHAR"/>
<result column="ORDER_DATE" property="orderDate" javaType="java.util.Date" jdbcType="DATE"/>
<result column="ORDER_TOTAL" property="orderTotal" javaType="java.math.BigDecimal" jdbcType="INTEGER"/>
<result column="PAYMENT_STATE" property="paymentState" javaType="int" jdbcType="INTEGER"/>
<result column="SEND_STATE" property="sendState" javaType="int" jdbcType="INTEGER"/> <association property="customer" column="ORDER_CUSTOMER_ID" javaType="customer" resultMap="customerMapper"  />
</resultMap> <select id="selectOrdersList" resultMap="orderMapper" >
  select 
c.customer_id as customerId,
c.customer_name as customerName,
o.ORDER_ID as orderId,
o.ORDER_NUMBER as orderNumber,
o.ORDER_DATE as orderDate,
o.ORDER_TOTAL as orderTotal,
o.PAYMENT_STATE as paymentState,
o.SEND_STATE as sendState
from t_orders o,t_customers c
where o.ORDER_CUSTOMER_ID=c.customer_id 
</select>
</mapper>
然后再创建SqlSession调用selectList("selectOrdersList")方法，我就不贴代码了，返回一个List<Order>,但是返回的数据我通过log4g的日志可以看到：返回的结果正常:
3:44:01,015 DEBUG ResultSet:27 - <==    Columns: customerId, customerName, orderId, orderNumber, orderDate, orderTotal, paymentState, sendState
13:44:01,031 DEBUG ResultSet:27 - <==        Row: 1, 张三, 1, 201009013-A00001, 2010-09-13, 500.0, 1, 0
13:44:01,031 DEBUG ResultSet:27 - <==        Row: 1, 张三, 6, 2010-0003, 2010-09-13, 655.0, 0, 1
13:44:01,031 DEBUG ResultSet:27 - <==        Row: 2, 李四, 7, 2010-0004, 2010-09-13, 700.66, 1, 0
13:44:01,031 DEBUG ResultSet:27 - <==        Row: 2, 李四, 8, 101009015-a00002, 2010-09-13, 800.0, 1, 1
---------------------------------
我的数据库里t_orders有四条记录，t_customers有2两条记录,从log4j的日志中可以看出查询数据和返回数据都很正常，而且也没有N+1问题，但是返回的List<Order>中的每个元素Order，只包括orderId,orderNumber,orderDate,orderTotal,paymentState,sendState;而并不包括customer属性，这样我就无法把张三和李四的名字通过order.getCustomer.getCustomerName()取出，因为order.getCustomer是Null值，请问我该怎么办？我实在没有分了，望不吝指教！谢谢！

解决方案 »

  1.   

    请问你那个log4j是如何配置的，我这里mybatis日志打印不了出来，其它都很正常，我们一起探讨！
      

  2.   

    order.getCustomer是Null值,原因如下：
    <resultMap type="customer" id="customerMapper">
            <id property="customerId" column="customer_id" javaType="long" jdbcType="INTEGER"/>
            <result property="customerName" column="customer_name" javaType="java.lang.String" jdbcType="VARCHAR"/>
            <result property="customerAddress" column="customer_address" javaType="java.lang.String" jdbcType="VARCHAR"/>
        </resultMap> <select id="selectOrdersList" resultMap="orderMapper" >
             select 
            c.customer_id as customerId,
            c.customer_name as customerName,
            o.ORDER_ID as orderId,
            o.ORDER_NUMBER as orderNumber,
            o.ORDER_DATE as orderDate,
            o.ORDER_TOTAL as orderTotal,
            o.PAYMENT_STATE as paymentState,
            o.SEND_STATE as sendState
            from t_orders o,t_customers c
            where o.ORDER_CUSTOMER_ID=c.customer_id 
        </select>

    红色部分标出了不匹配之处，根源是column的值要与sql语句里别名一致，不是与数据库表的列名一致，另外customer_address在查询语句里未出现，就没有构成customer的信息。