MyBatis：关联映射(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