Mybatis的映射关系配置

一，一对一

一对一映射是指一个对象与另一个对象具有一对一的关系

1.1 表关系

         一对一映射是指一个对象与另一个对象具有一对一的关系。例如，一个用户（User）与一个地址（Address）之间的关系。假设我们有以下表结构：

用户表：

id (int)
name (varchar)
address_id (int)

 地址表：

id (int)
street (varchar)
city (varchar)

 首先，创建 User 和 Address 实体类：

User.class

public class User {
    private int id;
    private String name;
    private Address address;
    // getters and setters
}

Address.class

public class Address {
    private int id;
    private String street;
    private String city;
    // getters and setters
}

接下来，创建一个 resultMap 进行一对一映射：

1.2 resultMap设置自定义映射

UserMapper.xml

<resultMap id="UserAddressResultMap" type="User">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    //一对一关系使用association
    <association property="address" javaType="Address">
        <id property="id" column="address_id"/>
        <result property="street" column="street"/>
        <result property="city" column="city"/>
    </association>
</resultMap>

属性：

id：表示自定义映射的唯一标识
type：查询的数据要映射的实体类的类型
子标签：

id：设置主键的映射关系
result：设置普通字段的映射关系
association ：设置多对一的映射关系
collection：设置一对多的映射关系
子标签属性：

property：设置映射关系中实体类中的属性名
column：设置映射关系中表中的字段名
最后，编写一个查询方法来使用这个 resultMap： 

UserMapper.xml

<select id="findUserWithAddress" resultMap="UserAddressResultMap">
    SELECT u.id, u.name, a.id as address_id, a.street, a.city
    FROM user u
    INNER JOIN address a ON u.address_id = a.id
    WHERE u.id = #{id}
</select>
        最后实现接口findUserWithAddress方法测试即可，通过以上简单的案例，我相信你已经明白自定义关系映射了。往往实际开发中数据和表是要复杂的多，进阶用法请看以下示例：
 

二、一对多映射（One-to-Many）

        一对多映射是指一个对象与多个对象具有一对多的关系。例如，一个订单（Oeder）与一个订单详情（OrderItem）之间的关系。假设我们有以下表结构：

 

 

2.1 创建实体

 利用mybatis逆向工程（generatorConfig.xml）生成模型层代码 :

package com.xzs.model;

import lombok.ToString;

@ToString
public class order {
    private Integer orderId;

    private String orderNo;

    public order(Integer orderId, String orderNo) {
        this.orderId = orderId;
        this.orderNo = orderNo;
    }

    public order() {
        super();
    }

    public Integer getOrderId() {
        return orderId;
    }

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

    public String getOrderNo() {
        return orderNo;
    }

    public void setOrderNo(String orderNo) {
        this.orderNo = orderNo;
    }
}

package com.xzs.model;

import lombok.ToString;

@ToString
public class orderItem {
    private Integer orderItemId;

    private Integer productId;

    private Integer quantity;

    private Integer oid;

    public orderItem(Integer orderItemId, Integer productId, Integer quantity, Integer oid) {
        this.orderItemId = orderItemId;
        this.productId = productId;
        this.quantity = quantity;
        this.oid = oid;
    }

    public orderItem() {
        super();
    }

    public Integer getOrderItemId() {
        return orderItemId;
    }

    public void setOrderItemId(Integer orderItemId) {
        this.orderItemId = orderItemId;
    }

    public Integer getProductId() {
        return productId;
    }

    public void setProductId(Integer productId) {
        this.productId = productId;
    }

    public Integer getQuantity() {
        return quantity;
    }

    public void setQuantity(Integer quantity) {
        this.quantity = quantity;
    }

    public Integer getOid() {
        return oid;
    }

    public void setOid(Integer oid) {
        this.oid = oid;
    }
}

创建 OrderVo 它是一个值对象（Value Object），继承 Order 类并添加了一个额外的属性 orderItems。该类用于在应用程序的各个层之间传递数据，尤其是在表示层和业务逻辑层之间。在这个例子中，OrderVo 类用于表示一个订单及其关联的订单项。

package com.xzs.vo;
 
import com.xzs.model.Order;
import com.xzs.model.OrderItem;
import lombok.Data;
 
import java.util.ArrayList;
import java.util.List;
 

@Data
public class OrderVo extends Order {
    private List<OrderItem> orderItems = new ArrayList<>();
}

2.2 级联方式处理映射关系

    <resultMap id="OrderVoMap" type="com.xzs.vo.OrderVo">
        <result column="order_id" property="orderId"></result>
        <result column="order_no" property="orderNo"></result>
        //多关系使用collection
        <collection property="orderItems" ofType="com.xzs.model.OrderItem">
            <result column="order_item_id" property="orderItemId"></result>
            <result column="product_id" property="productId"></result>
            <result column="quantity" property="quantity"></result>
            <result column="oid" property="oid"></result>
        </collection>
    </resultMap>

2.3 定义SQL

    <select id="SelectByOid" resultMap="OrderVoMap" parameterType="java.lang.Integer">
        SELECT *
        FROM t_hibernate_order o,
             t_hibernate_order_item oi
        WHERE o.order_id = oi.oid
          AND o.order_id = #{oid}
    </select>

2.4 OrderMapper接口

package com.xzs.mapper;
 
import com.xzs.model.Order;
import com.xzs.vo.OrderVo;
import org.apache.ibatis.annotations.Param;
import org.springframework.stereotype.Repository;
 
@Repository
public interface OrderMapper {
 
    OrderVo SelectByOid(@Param("oid") Integer oid);
}

2.5 编写业务逻辑层

OrderItmeBiz 接口

package com.xzs.biz;
 
import com.xzs.vo.OrderItemVo;
 

public interface OrderItmeBiz {
    OrderItemVo SelectByOitemId(Integer oid);
}

OrderBizImpl 接口实现类

package com.xzs.biz.impl;
 
import com.xzs.biz.OrderBiz;
import com.xzs.mapper.OrderMapper;
import com.xzs.vo.OrderVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
 

@Service
public class OrderBizImpl implements OrderBiz {
    @Autowired
    private OrderMapper orderMapper;
 
    @Override
    public OrderVo SelectByOid(Integer oid) {
        return orderMapper.SelectByOid(oid);
    }
}

三、多对多映射（Many-to-Many）

3.1 表关系
        多对多映射是指多个对象与多个对象具有多对多的关系。表之间的多对多关系稍微复杂，需要一个中间表来表示：

        中间表有三个字段，其中两个字段分别作为外键指向各自一方的主键，由此通过中间表来表示多对多关系，通过一个表联合另一个中间表可以表示为一对多关系。

 

 

如：根据书籍id查找关联属性类别：

3.2 创建实体

利用mybatis逆向工程（generatorConfig.xml）生成模型层代码 :

package com.xzs.model;

import lombok.ToString;

@ToString
public class HCategory {
    private Integer categoryId;

    private String categoryName;

    public HCategory(Integer categoryId, String categoryName) {
        this.categoryId = categoryId;
        this.categoryName = categoryName;
    }

    public HCategory() {
        super();
    }

    public Integer getCategoryId() {
        return categoryId;
    }

    public void setCategoryId(Integer categoryId) {
        this.categoryId = categoryId;
    }

    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }
}

package com.xzs.model;

import lombok.ToString;

@ToString
public class HBookCategory {
    private Integer bcid;

    private Integer bid;

    private Integer cid;

    public HBookCategory(Integer bcid, Integer bid, Integer cid) {
        this.bcid = bcid;
        this.bid = bid;
        this.cid = cid;
    }

    public HBookCategory() {
        super();
    }

    public Integer getBcid() {
        return bcid;
    }

    public void setBcid(Integer bcid) {
        this.bcid = bcid;
    }

    public Integer getBid() {
        return bid;
    }

    public void setBid(Integer bid) {
        this.bid = bid;
    }

    public Integer getCid() {
        return cid;
    }

    public void setCid(Integer cid) {
        this.cid = cid;
    }
}

package com.xzs.model;

import lombok.ToString;

@ToString
public class HBook {
    private Integer bookId;

    private String bookName;

    private Float price;

    public HBook(Integer bookId, String bookName, Float price) {
        this.bookId = bookId;
        this.bookName = bookName;
        this.price = price;
    }

    public HBook() {
        super();
    }

    public Integer getBookId() {
        return bookId;
    }

    public void setBookId(Integer bookId) {
        this.bookId = bookId;
    }

    public String getBookName() {
        return bookName;
    }

    public void setBookName(String bookName) {
        this.bookName = bookName;
    }

    public Float getPrice() {
        return price;
    }

    public void setPrice(Float price) {
        this.price = price;
    }
}

创建 HBookVo 值对象（Value Object）

package com.xzs.vo;
 
import com.xzs.model.BookCategory;
import com.xzs.model.HBook;
import lombok.Data;
 
import java.util.List;
 

@Data
public class HBookVo extends HBook {
    private List<BookCategory> bookc = new ArrayList<>();
}

3.3 处理映射关系、定义sql

  <resultMap id="HBookVoMap" type="com.xzs.vo.HBookVo" >
    <result column="book_id" property="bookId"></result>
    <result column="book_name" property="bookName"></result>
    <result column="price" property="price"></result>
    <collection property="bookc" ofType="com.xzs.model.Category">
      <result column="category_id" property="categoryId"></result>
      <result column="category_name" property="categoryName"></result>
    </collection>
  </resultMap>
 
  <!--根据书籍的id查询书籍的信息及所属属性-->
  <select id="selectByBookId" resultMap="HBookVoMap" parameterType="java.lang.Integer">
    SELECT
      *
    FROM
      t_hibernate_book b,
      t_hibernate_category c,
      t_hibernate_book_category bc
    WHERE
      b.book_id = bc.bid
      AND c.category_id = bc.bcid
      AND b.book_id = #{bid}
  </select>

3.4 HBookMapper 接口

HBookVo selectByBookId(@Param("bid") Integer bid);

3.5 编写业务逻辑层

HBookBiz 接口

package com.xzs.biz;
 
import com.xzs.vo.HBookVo;
 

public interface HBookBiz {
    HBookVo selectByBookId(Integer bid);
}

HBookBizImpl 接口实现类

package com.xzs.biz.impl;
 
import com.xzs.biz.HBookBiz;
import com.xzs.mapper.HBookMapper;
import com.xzs.vo.HBookVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
 
@Service
public class HBookBizImpl implements HBookBiz {
    @Autowired
    private HBookMapper hBookMapper;
    @Override
    public HBookVo selectByBookId(Integer bid) {
        return hBookMapper.selectByBookId(bid);
    }
}