MyBatis关联映射之一对一，一对多，多对多

　　实现项目开发中，存在一对一，一对多，多对多的映射关系，Mybatis对这些映射关系提供了完全的支持。

一对一

一对一关系：一个用户只有一个身份证号码；在数据库上可以建立主外键关联，增加外键唯一约束。

1. com.entity.Card.java

    public class Card{
        private Integer id;
        private String code;
   
        //---------set/get方法------------
    }
   

2. com.entity.User.java

    public class User{
        private Integer id;
        private String name;
        private String sex;
        private Integer age;
        //身份证号
        private Card card;
   
        //---------set/get方法------------
    }
   

3. com.mapper.CardMapper.xml

    <!-- 根据ID查询身份证 -->
    <select id="selectCardById" parameterType="int" resultType="com.entity.Card">
        select * from card where id = #{id}
    </select>
   

4. com.mapper.UserMapper.xml

    <mapper namespace="com.mapper.UserMapper">
        <select id="selectUserById" parameterType="int" resultMap="userMapper">
            select * from uer where id = #{id}
        </select>
        <resultMap type="com.entity.User" id="userMapper">
            <id property="id" column="id" />
            <result property="name" column="name"/>
            <result property="sex" column="sex"/>
            <result property="age" column="age"/>
            <association property="card" column="card_id" 
                select="com.mapper.CardMapper.selectCardById" 
                javaType="com.entity.Card"/>
        </resultMap>
    </mapper>
   

5. 上面例子解读

   • 根据用户ID查询用户信息，用户信息包含了用户身份证信息(对象)，在查用户信息时，返回的结果类型resultMap，名为userMapper；
   • 查找ID名为userMapper的 resultMap，里面有字段映射；Card字段使用的是一对一的关联映射association；调用了DAO里的com.mapper.CardMapper.selectCardById接口方法来查询。

6. 注解方式实现一对一关系映射

    /**
    * 一对一关联注解：需要关联执行的SQL语句
    * fetchType表示查询时是立即加载(eager)还是懒加载(lazy)
    * @param id
    * @return
    */
    @Select("select * from user where id = #{id}")
    @Results({
       @Result(id=true,column="id",property="id"),
       @Result(column="name",property="name"),
       @Result(column="sex",property="sex"),
       @Result(column="age",property="age"),
       @Result(column="card_id",property="card",
      one=@One(select="com.vending.mapper.CardMapper.selectCardById",
       fetchType=FetchType.EAGER))})
    User selectUserById(Integer id);
   

一对多

一对多关系：一个用户有多个订单，一个订单只属于一个用户；数据库可以使用主外键关联，外键在多方，多方维护关系。

1. com.entity.User

    public class User{
   
        private Integer id;
        private String name;
        private List<Order> orderList;
   
        //------set/get方法---------
    }
   

2. com.entity.Order

    public class Order{
   
        private Integer id;
        private User user;
        private Double totalPrice;
        private Date createDateTime;
   
        //------set/get方法---------
    }
   

3. com.mapper.UserMapper.xml

    <mapper namespace="com.mapper.UserMapper">
   
        <!-- 根据用户ID查询订单信息，返回resultMap -->
        <select id="selectUserById" parameterType="int" resultMap="userResultMap">
            select * from user where id = #{id}
        </select>
        <resultMap type="com.entity.User" id="userResultMap">
            <id property="id" column="id"/>
            <result property="name" column="name"/>
            <!-- 一对多关系,集合使用collection,
                fetchType="lazy"表示懒加载，正常情况下，一对多所关联的集合对象，
                    应设置成lazy,需要用到该属性时再去加载。            
             -->
            <collection property="orderList" javaType="ArrayList" 
                column="id" ofType="com.entity.Order"
                select="com.mapper.OrderMapper.selectOrderByUserId" fetchType="lazy">
                <id property="id" column="id"/>
                <result property="totalPrice" column="totalPrice"/>
                <result property="createDateTime" column="createDateTime"/>
            </collection>
        </resultMap>
    </mapper>
   

4. com.mapper.OrderMapper.xml

    <mapper namespace="com.mapper.OrderMapper">
   
        <!-- 根据ID查询订单信息 -->
        <select id="selectOderById" parameterType="int" resultMap="orderResultMap"> 
            select * from order o, user u
                where o.user_id = u.id
                    and o.id = #{id}
        </select>
   
        <!-- 根据用户ID查询订单信息，返回resultMap -->
        <select id="selectOrderByUserId" parameterType="int" resultMap="orderResultMap">
            select * from order where user_id = #{id}
        </select>
   
        <!-- 映射Order对象的reultMap -->
        <resultMap type="com.entity.Order" id="orderResultMap">
            <id property="id" column="id"/>
            <result property="totalPrice" column="totalPrice"/>
            <result property="createDateTime" column="createDateTime"/>
            <!-- 一个订单只属于一个用户，一对一，使用association -->
            <association property="user" javaType="com.entity.User">
                <id property="id" column="id"/>
                <result property="name" column="name"/>
            </association>
        </resultMap>
   
    </mapper>
   

多对多

多对多关系：一个订单中可以购买多种商品，一种商品可以被多个订单购买；多对多关系采用中间表来维护。

1. com.entity.order

    public class Order{
        private Integer id;
        private Double total;
        private List<Goods> goodList;    
    }
   

2. com.entity.Goods

    public class Goods{
        private Integer id;
        private String name;
        private Double price;
        private String remark;
        private List<Order> orderList;    
    }
   

3. com.mapper.OrderMapper.xml

    <mapper namespace="com.mapper.OrderMapper">
        <select id="selectOrderById" parameterType="int" resultMap="orderResultMap">
            select o.id as orderId from order o where id = #{id}
        </select>
        <resultMap type="com.entity.Order" id="orderResultMap">
            <id property="id" column="id"/>
            <result property="total" column="total"/>
            <!-- 多对多映射：collection -->
            <collection property="goodList" javaType="ArrayList"
                column="orderId" ofType="com.entity.Goods"
                select="com.mapper.GoodsMapper.selectGoodsListByOrderId">
                <id property="id" column="id"/>
                <result property="name" column="name"/>
                <result property="price" column="price"/>
                <result property="remark" column="remark"/>
            </collection>
        </resultMap>    
    </mapper>
   

4. com.mapper.GoodsMapper.xml
   内容同上