Mybatis——关联映射查询

一、一对一映射：

主Model：

package com.neuedu.model;

import java.util.Date;

/**
 * orders订单表
 */
public class Order {
    
    private int id;
    
    private int userid;
    
    private String orderId;
    
    private Date createTime;
    
    private String note;
    
    //一对一关系
    private User user;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getUserid() {
        return userid;
    }

    public void setUserid(int userid) {
        this.userid = userid;
    }

    public String getOrderId() {
        return orderId;
    }

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

    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;
    }
    
    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    @Override
    public String toString() {
        return "Order [id=" + id + ", userid=" + userid + ", orderId=" + orderId + ", createTime=" + createTime
                + ", note=" + note + ", user=" + user + "]";
    }
    
}

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

package com.neuedu.model;
/**
 * 自定义
 */
public class OrderCustomer extends Order{

    private int userid;
    
    private String username;
    
    private String address;

    public int getUserid() {
        return userid;
    }

    public void setUserid(int userid) {
        this.userid = userid;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "OrderCustomer [userid=" + userid + ", username=" + username + ", address=" + address + ", getId()="
                + getId() + ", getOrderId()=" + getOrderId() + ", getCreateTime()=" + getCreateTime() + ", getNote()="
                + getNote() + "]";
    }
    
}

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

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

<!-- 一对一查询，返回resultMap -->
    <resultMap id="selectOrderResultMap" type="com.neuedu.model.Order" >
        <id property="id" column="id"/>    <!-- property pojo对象的属性名称    column查询语句的列名称-->
        <result property="userid" column="userid"/>
        <result property="orderId" column="orderid"/>
        <result property="createTime" column="createtime"/>
        <result property="note" column="note"/>
        <!-- 一对一 关系-->
        <association property="user" javaType="com.neuedu.model.User">
            <id property="id" column="userid"/>
            <result property="username" column="username"/>
            <result property="birthday" column="birthday"/>
            <result property="address" column="address"/>
        </association>
    </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

<select id="selectOrderByResultMap" resultMap="selectOrderResultMap">
        select 
            o.id, o.userid, o.orderid, o.createtime, o.note,
            t.id userid,t.username,t.birthday,t.address 
        from orders o,t_user t 
        where o.userid = t.id
    </select>

二、一对多查询：

主Model中添加属性

//一对多关系
    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

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