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