mybatis之一个实体类内部嵌套两个同类型对象

说在前面

这个问题不知道其他人会不会遇见，反正我遇见了，我不管，我就是要记录下来，谁知道我下次会不会再遇见，毕竟我健忘呀。

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问题描述

昨天做项目的时候要实现一个需求的的功能，持久层里要添加查找需求单的详细信息的接口，注意哦，是同时要查找出发单人还有接单人这两个对象的姓名、头像的哦，而发单人这个对象和接单人对象的关系是外键连接。

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老规矩，以源码为导向

package ssm.aidai.pojo;

Order 实体类

import java.math.BigDecimal;
import java.util.Date;

public class Order {

private Long orderId;

private BigDecimal orderReward;

private Integer orderReceiveId;

private Integer orderType;

private Integer orderSendId;

private Integer placeId;

private Date orderTime;

private String orderExchnPlace;

private Date orderExpireTime;

private Date orderSendTime;

private BigDecimal orderWyMoney;

private Integer orderStatus;

private Date orderFinishTime;

private String orderContent;

private User receiveUser; //接单人User

private User sendUser;//发单人User

private Place place;


public User getReceiveUser() {
    return receiveUser;
}

public void setReceiveUser(User receiveUser) {
    this.receiveUser = receiveUser;
}

public User getSendUser() {
    return sendUser;
}

public void setSendUser(User sendUser) {
    this.sendUser = sendUser;
}

public Place getPlace() {
    return place;
}

public void setPlace(Place place) {
    this.place = place;
}

public Long getOrderId() {
    return orderId;
}

public void setOrderId(Long orderId) {
    this.orderId = orderId;
}

public BigDecimal getOrderReward() {
    return orderReward;
}

public void setOrderReward(BigDecimal orderReward) {
    this.orderReward = orderReward;
}

public Integer getOrderReceiveId() {
    return orderReceiveId;
}

public void setOrderReceiveId(Integer orderReceiveId) {
    this.orderReceiveId = orderReceiveId;
}

public Integer getOrderType() {
    return orderType;
}

public void setOrderType(Integer orderType) {
    this.orderType = orderType;
}

public Integer getOrderSendId() {
    return orderSendId;
}

public void setOrderSendId(Integer orderSendId) {
    this.orderSendId = orderSendId;
}

public Integer getPlaceId() {
    return placeId;
}

public void setPlaceId(Integer placeId) {
    this.placeId = placeId;
}

public Date getOrderTime() {
    return orderTime;
}

public void setOrderTime(Date orderTime) {
    this.orderTime = orderTime;
}

public String getOrderExchnPlace() {
    return orderExchnPlace;
}

public void setOrderExchnPlace(String orderExchnPlace) {
    this.orderExchnPlace = orderExchnPlace == null ? null : orderExchnPlace
            .trim();
}

public Date getOrderExpireTime() {
    return orderExpireTime;
}

public void setOrderExpireTime(Date orderExpireTime) {
    this.orderExpireTime = orderExpireTime;
}

public Date getOrderSendTime() {
    return orderSendTime;
}

public void setOrderSendTime(Date orderSendTime) {
    this.orderSendTime = orderSendTime;
}

public BigDecimal getOrderWyMoney() {
    return orderWyMoney;
}

public void setOrderWyMoney(BigDecimal orderWyMoney) {
    this.orderWyMoney = orderWyMoney;
}

public Integer getOrderStatus() {
    return orderStatus;
}

public void setOrderStatus(Integer orderStatus) {
    this.orderStatus = orderStatus;
}

public Date getOrderFinishTime() {
    return orderFinishTime;
}

public void setOrderFinishTime(Date orderFinishTime) {
    this.orderFinishTime = orderFinishTime;
}

public String getOrderContent() {
    return orderContent;
}

public void setOrderContent(String orderContent) {
    this.orderContent = orderContent == null ? null : orderContent.trim();
 }
}

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OrderMapper关键源码：

<resultMap id="BaseResultMap2" type="ssm.aidai.pojo.Order"
    extends="BaseResultMap">
    <!-- 接单人身份的receiveUser关联 -->
    <association property="receiveUser" javaType="ssm.aidai.pojo.User"
        column="order_receive_id" select="getReceiveUser" />
    <!-- 发单人身份的sendUser关联 -->
    <association property="sendUser" column="order_send_id"
        select="getSendUser" />
</resultMap>
<!-- 连接接单人对象的函数 -->
<select id="getReceiveUser" resultMap="UserResultMap"
    parameterType="java.lang.Integer">
    select
    user_id,user_name,user_icon
    from aidai_user where
    user_id =
    #{orderReceiveId,jdbcType=INTEGER}
</select>
<!-- 连接发单人对象的函数 -->
<select id="getSendUser" resultMap="UserResultMap"
    parameterType="java.lang.Integer">
    select
    user_id,user_name,user_icon
    from aidai_user where
    user_id =
    #{orderSendId,jdbcType=INTEGER}
</select>
<!-- 查找订单信息 -->
<select id="selectByPrimaryKey" resultMap="BaseResultMap2"
    parameterType="java.lang.Long">
    select
    <include refid="Base_Column_List" />
    ,
    <include refid="Additional_Column_Place" />
    from aidai_order o,
    aidai_place p
    where order_id =
    #{orderId,jdbcType=INTEGER} and p.place_id =
    o.place_id
</select>

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源码分析:

在Order实体类中嵌套这两个对象，分别是：receiveUser和sendUser，而我们要做的就是将在查询Order这个对象的时候通过orderReceiveId和orderSendId这两个外键查询receiveUser和sendUser这两个对象，那么我是怎么做的呢？
在resultMap中：

      <resultMap id="BaseResultMap2" type="ssm.aidai.pojo.Order" extends="BaseResultMap"> 
      <!-- 接单人身份的receiveUser关联 --> 
      <association property="receiveUser" javaType="ssm.aidai.pojo.User" column="order_receive_id" select="getReceiveUser" /> 
       <!-- 发单人身份的sendUser关联 --> 
      <association property="sendUser" column="order_send_id"   javaType="ssm.aidai.pojo.User"  select="getSendUser" />
      </resultMap>

根据应用场景进行元素分析：

• association： 复杂对象的映射
• property： 映射到Order实体类中的receiveUser
• column ： 这个其实就是你要传的参数名
• javaType： 一个 Java 类的完全限定名

整个查询操作流程是这样的：

1、数据库依据下面的代码查找Order对象，然后将对象映射到BaseResultMap2结果集中；

  <select id="selectByPrimaryKey" resultMap="BaseResultMap2"
    parameterType="java.lang.Long">
    select
    <include refid="Base_Column_List" />
    ,
    <include refid="Additional_Column_Place" />
    from aidai_order o,
    aidai_place p
    where order_id =
    #{orderId,jdbcType=INTEGER} and p.place_id =
    o.place_id
</select>

2、而在BaseResultMap2中有以下association复杂对象的映射，即

    <association property="receiveUser" javaType="ssm.aidai.pojo.User" column="order_receive_id" select="getReceiveUser" /> 

所以系统会通过select的属性值即getReceiveUser跳转到id="getReceiveUser"的xml中，并将column中的order_receive_id的映射字段中的值传递过去即

  <select id="getReceiveUser" resultMap="UserResultMap"
    parameterType="java.lang.Integer">
    select
    user_id,user_name,user_icon
    from aidai_user where
    user_id =
    #{orderReceiveId,jdbcType=INTEGER}
  </select>

那么为什么是#{orderReceiveId,jdbcType=INTEGER}呢，因为orderReceiveId是order_receive_id在实体类Order中的映射字段，可以装载值。
在查询到对象之后会将对象放入receiveUser。
sendUser的操作是一样的，就不解释了！！！

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Note:发布的这些文章全都是自己边学边总结的，难免有纰漏，如果发现有不足的地方，希望可以指出来，一起学习咯，么么哒。
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