Mybatis关联映射之association和collection

最近在学习中，遇到一个混淆点，写篇文章记下来。在Mybatis中使用select语句时，会使用association和collection进行映射。两者的区别主要是，association用于一对一，即一个实体类对象是另一个实体类对象的属性；collection用于一对多，例如一个实体类对象里有一个集合作为属性。

举一个实例。

一辆车只有一个车主，而车主可以拥有好几辆车。

汽车实体类：

public class CarBean {
    private int id;
    private String type;
    private int price;
    private ManBean man;

车主实体类：

public class ManBean {
    private int id;
    private String name;
    private LocalDate birthday;
    private int carNum;
    private List<CarBean> carList;

1、根据汽车id查询汽车，同时需要查询该汽车的车主。

    <select id="findById" resultMap="carMap">
        select * from t_car where pk_carId = #{id};
    </select>

    <resultMap id="carMap" type="CarBean">
        <id column="pk_carId" property="id"></id>
        <result column="c_type" property="type"></result>
        <result column="c_price" property="price"></result>
        <result column="m_name" property="man.name"></result>
        <!--property为关联属性，select表示该属性的值来自哪个语句块查询结果，column表示该语句块传递哪个列的值-->
        <association property="man" select="findMan" column="fk_manId"></association>
    </resultMap>

    <select id="findMan" resultMap="manMap">
        select * from t_man where pk_manId = #{id};
    </select>

    <resultMap id="manMap" type="ManBean">
        <id property="id" column="pk_manId"></id>
        <result property="name" column="m_name"></result>
        <result property="birthday" column="m_birthday"></result>
    </resultMap>

2、根据车主id查询车主，同时需要查询该车主的所有汽车

①关联语句块

   <select id="findById" resultMap="manMap">
      select * from t_man where pk_manId = #{id};
   </select>

   <resultMap id="manMap" type="ManBean">
      <id column="pk_manId" property="id"></id>
      <result property="name" column="m_name"></result>
      <result property="birthday" column="m_birthday"></result>
      <result property="carNum" column="carNum"></result>
      <collection property="carList" select="findCar" column="pk_manId"></collection>
   </resultMap>

   <select id="findCar" resultMap="carMap">
      select * from t_car where fk_manId = #{id};
   </select>

   <resultMap id="carMap" type="CarBean">
      <id property="id" column="pk_carId"></id>
      <result property="type" column="c_type"></result>
      <result property="price" column="c_price"></result>
   </resultMap>

②连表查询

   <resultMap id="manMap" type="ManBean">
      <id column="pk_manId" property="id"></id>
      <result property="name" column="m_name"></result>
      <result property="birthday" column="m_birthday"></result>
      <result property="carNum" column="carNum"></result>
      <!--对集合进行映射，ofType指定集合中存放元素的类型-->
      <collection property="carList" ofType="CarBean">
         <id property="id" column="pk_carId"></id>
         <result property="type" column="c_type"></result>
         <result property="price" column="c_price"></result>
      </collection>
   </resultMap>

   <select id="findById" resultMap="manMap">
      SELECT * FROM t_man m LEFT JOIN t_car c ON m.`pk_manId` = c.`fk_manId` WHERE m.`pk_manId` = #{id};
   </select>