Mybatis中对象关系映射_mybatis映射对象

在实际开发中，实体类之间有一对一、一对多、多对多的关系，所以需要正确配置它们对应关系，Mybatis通过配置文件能够从数据库中获取列数据后自动封装成对象。

如：一个订单Orders类对应一个用户User类，一个User类对应多个Orders类。
Orders.java

package cn.itheima.pojo;
import java.util.Date;

public class Orders {
private Integer id;

private Integer userId;

private String number;

private Date createtime;

private String note;

private User user;  // 一个订单对应一个用户

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;
}  
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}

User.java

package cn.itheima.pojo;
import java.util.Date;
import java.util.List;

public class User {
private int id;
private String username;// 用户姓名
private String sex;// 性别
private Date birthday;// 生日
private String address;// 地址

private List<Orders> ordersList;  // 一个用户对应多个订单

public List<Orders> getOrdersList() {
    return ordersList;
}
public void setOrdersList(List<Orders> ordersList) {
    this.ordersList = 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 i) {
    this.sex = i;
}
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;
}
@Override
public String toString() {
    return "User [id=" + id + ", username=" + username + ", sex=" + sex
            + ", birthday=" + birthday + ", address=" + address + "]";
}   
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}

一、一对一映射

1、 在UserMapper.xml中配置Orders和User的一对一映射关系

<!-- 
    id：resultMap的唯一标识
    type：将查询出的数据放入到指定对象中
    注意：手动映射需要指定数据库中表的字段名与java中pojo类的属性名称对应关系
 -->
<resultMap type="cn.itheima.pojo.Orders" id="orderAndUserResultMap">
    <!-- 
        id：指定主键字段对应关系
        column：列，数据库中的字段名称
        property：属性，pojo的属性名称
     -->
    <id column="id" property="id"/>
    <!-- result：指定非主键字段的对应关系 -->
    <result column="user_id" property="userId"/>
    <result column="number" property="number"/>
    <result column="createtime" property="createtime"/>
    <result column="note" property="note"/>
    <!-- 指定单个对象的对应关系
        property：指定将数据放入Orders中的user属性中
        javaType：user属性的类型
     -->
    <association property="user" javaType="cn.itheima.pojo.User">
        <id column="uid" property="id"/>
        <result column="username" property="username"/>
        <result column="sex" property="sex"/>
        <result column="birthday" property="birthday"/>
        <result column="address" property="address"/>
    </association>
</resultMap>
<select id="findOrdersAndUser2" resultMap="orderAndUserResultMap">
    SELECT a.*,b.id uid,username,birthday,sex,address
    FROM orders a,user b
    WHERE a.user_id=b.id
</select>
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2、在UserMapper.java中编写接口

List findOrdersAndUser2();
3、测试

@Test
public void testFindOrdersAndUser2() throws Exception {
    SqlSession session = sqlSessionFactory.openSession();
    UserMapper userMapper = session.getMapper(UserMapper.class);
    List<Orders> list = userMapper.findOrdersAndUser2();
    System.out.println(list);
}
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二、一对多映射

1、在UserMapper.xml中配置User和Orders的一对多关系

<resultMap type="cn.itheima.pojo.User" id="userAndOrdersResultMap">
    <id column="id" property="id"/>
    <result column="birthday" property="birthday"/>
    <result column="username" property="username"/>
    <result column="sex" property="sex"/>
    <result column="address" property="address"/>
    <!-- 指定对应的集合对象关系映射
        property：将数据放入user对象中的ordersList中
        ofType：指定ordersList属性的泛型类型
     -->
    <collection property="ordersList" ofType="cn.itheima.pojo.Orders">
        <id column="oid" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="number" property="number"/>
        <result column="createtime" property="createtime"/>
    </collection>
</resultMap>
<select id="findUserAndOrders" resultMap="userAndOrdersResultMap">
    SELECT a.*,b.id oid,user_id,number,createtime 
    FROM user a,orders b WHERE a.id=b.user_id
</select>
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2、在UserMapper.java中编写接口

List findUserAndOrders();
3、测试

@Test
public void testFindUserAndOrders() throws Exception {
    SqlSession session = sqlSessionFactory.openSession();
    UserMapper userMapper = session.getMapper(UserMapper.class);
    List<User> list = userMapper.findUserAndOrders();
    System.out.println(list);
}
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