Original link: http://www.yiidian.com/mybatis/one-to-one.html

1 What are one-to-one, one-to-many mappings?

Take users and orders for example,

One-to-one: An order belongs to only one user==>The order is one-to-one with the user

One-to-many: A user can have multiple orders==>A user is one-to-many with an order

Note: In MyBatis, if you want to complete a many-to-many relationship, it is actually two one-to-many mappings!

Next, let's explain how MyBatis's one-to-one mapping works.

2 Establish table structure

2.1 Create User Table

CREATE TABLE `t_user` (
   `id` int(11) DEFAULT NULL,
   `username` varchar(50) DEFAULT NULL,
   `password` varchar(50) DEFAULT NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8

2.2 Create Order Form

CREATE TABLE `t_order` (
   `id` int(11) DEFAULT NULL,
   `orderno` varchar(100) DEFAULT NULL,
   `amount` double DEFAULT NULL,
   `user_id` int(11) DEFAULT NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8

2.3 Insert Test Data

3 Design Pojo entities and build relationships

3.1 User Entity Class

package com.yiidian.domain;

import java.util.List;

/**
 * User Entities
 * One Point Tutorial Web - www.yiidian.com
 */
public class User {
    private Integer id;
    private String username;
  
    public void setOrders(List<Order> orders) {
        this.orders = orders;
    }

    public Integer getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

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

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

3.2 Order Entity Class

package com.yiidian.domain;

/**
 * Order Entities
 * One Point Tutorial Web - www.yiidian.com
 */
public class Order {
    private Integer id;
    private String orderno;
    private Double amount;
    private User user;

    public Integer getId() {
        return id;
    }

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

    public String getOrderno() {
        return orderno;
    }

    public void setOrderno(String orderno) {
        this.orderno = orderno;
    }

    public Double getAmount() {
        return amount;
    }

    public void setAmount(Double amount) {
        this.amount = amount;
    }

    public User getUser() {
        return user;
    }

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

Order entity class that establishes a relationship with a User entity through the user attribute.Note that here is a User.

4 Write Dao interface

4.1 UserDao interface

package com.yiidian.dao;

import com.yiidian.domain.User;

import java.util.List;

/**
 * User Dao Interface
 * One Point Tutorial Web - www.yiidian.com
 */
public interface UserDao {

}

4.2 OrderDao interface

package com.yiidian.dao;
import com.yiidian.domain.Order;
import com.yiidian.domain.User;
import java.util.List;

/**
 * Order Dao Interface
 * One Point Tutorial Web - www.yiidian.com
 */
public interface OrderDao {
    /**
     * Query all orders
     */
    public List<Order> findAllOrders();
}

5 Write Dao Mapping Configuration

5.1 UserDao.xml Configuration

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--
   namespace: Used to specify that the mapping file needs to be mapped Dao Interface
-->
<mapper namespace="com.yiidian.dao.UserDao">

</mapper>

5.2 OrderDao.xml Configuration

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--
   namespace: Used to specify that the mapping file needs to be mapped Dao Interface
-->
<mapper namespace="com.yiidian.dao.OrderDao">

    <!--One-to-one mapping-->
    <resultMap id="OrderResultMap" type="com.yiidian.domain.Order">
        <id property="id" column="oid"/>
        <result property="orderno" column="orderno"/>
        <result property="amount" column="amount"/>
        <!--Associate the user to which the query order belongs-->
        <association property="user" column="id" javaType="com.yiidian.domain.User">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="password" column="password"/>
        </association>
    </resultMap>

    <select id="findAllOrders" resultMap="OrderResultMap">
        SELECT
          o.id oid,
          o.orderno orderno,
          o.amount amount,
          u.*
        FROM t_order o
          LEFT JOIN t_user u
            ON o.user_id = u.id
    </select>
</mapper>

• Association: used in one-to-one association mapping
• Property: the user property of the corresponding Order class
• column: corresponding foreign key field name
• javaType:Fully qualified name of User class

6 Write test classes

package com.yiidian.mybatis;

import com.yiidian.dao.OrderDao;
import com.yiidian.domain.Order;
import com.yiidian.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

/**
 * MyBatis Test Class-One-to-One Mapping
 * One Point Tutorial Web - www.yiidian.com
 */
public class TestOne2One {

    /**
     * Test one-to-one mapping
     */
    @Test
    public void testOrderDao(){
        //1. Get the SqlSession object
        SqlSession sqlSession = MyBatisUtils.getSession();

        //2. Create Mapper Proxy Object
        OrderDao orderDao = sqlSession.getMapper(OrderDao.class);

        //3. Call Method
        List<Order> list = orderDao.findAllOrders();
        System.out.println(list);
        //4. Close the connection
        sqlSession.close();
    }

}

7 Run Test Class

Start the test class in debug mode and look at the list variable to see that the Order object successfully encapsulates the User object's data!

Source download: https://pan.baidu.com/s/1jZrfapjqB_VHI_GLgKPo4g