Mybatis 示例之 Association - 偶尔记一下 - 博客频道 - CSDN.NET

接下来的文章中，关于Mybatis的示例，全部来自于Mybatis代码中的单元测试代码，通过这些代码能够学习Mybatis中很有用的知识， 这些内容在doc文档中可能只是简单提到了，或者有一些文字说明，通过这些单元测试能更直观的了解如何在Mybatis使用这些内容。

这一节内容为Association关联的结果查询，就是在查询出结果后，根据查询的列和resultMap定义的对应关系，来创建对象并写入值。

• association?– 一个复杂的类型关联;许多结果将包成这种类型
  • 嵌入结果映射 – 结果映射自身的关联,或者参考一个

（注：“参考一个”，这里参考一个是通过对象的Key来唯一确定的，如果Key值一样，就直接用已经存在的这个对象。）

association是resultMap中的一个配置选项，下面是用到的类的UML图：

Car对象中包含了Engine和Brakes两个对象。Mapper是接口对象。AssociationTest是该测试对象。

SQL表结构和数据：

[sql] view plaincopyprint?

1. drop?table?cars?if?exists;??
2. ??
3. create?table?cars?(??
4. ??carid?integer,??
5. ??cartype?varchar(20),??
6. ??enginetype?varchar(20),??
7. ??enginecylinders?integer,??
8. ??brakestype?varchar(20)??
9. );??
10. ??
11. insert?into?cars?(carid,?cartype,?enginetype,?enginecylinders,?brakestype)?values(1,?‘VW‘,???‘Diesel‘,?4,????null);??
12. insert?into?cars?(carid,?cartype,?enginetype,?enginecylinders,?brakestype)?values(2,?‘Opel‘,????null,????null,?‘drum‘);??
13. insert?into?cars?(carid,?cartype,?enginetype,?enginecylinders,?brakestype)?values(3,?‘Audi‘,?‘Diesel‘,?4,????‘disk‘);??
14. insert?into?cars?(carid,?cartype,?enginetype,?enginecylinders,?brakestype)?values(4,?‘Ford‘,?‘Gas‘,????8,????‘drum‘);??

drop table cars if exists;

create table cars (
  carid integer,
  cartype varchar(20),
  enginetype varchar(20),
  enginecylinders integer,
  brakestype varchar(20)
);

insert into cars (carid, cartype, enginetype, enginecylinders, brakestype) values(1, ‘VW‘,   ‘Diesel‘, 4,    null);
insert into cars (carid, cartype, enginetype, enginecylinders, brakestype) values(2, ‘Opel‘,    null,    null, ‘drum‘);
insert into cars (carid, cartype, enginetype, enginecylinders, brakestype) values(3, ‘Audi‘, ‘Diesel‘, 4,    ‘disk‘);
insert into cars (carid, cartype, enginetype, enginecylinders, brakestype) values(4, ‘Ford‘, ‘Gas‘,    8,    ‘drum‘);

[html] view plaincopyprint?

1. <mapper?namespace="org.apache.ibatis.submitted.associationtest.Mapper">??
2. ??
3. ????<resultMap?type="org.apache.ibatis.submitted.associationtest.Car"?id="carResult">??
4. ????????<id?column="carid"?property="id"/>??
5. ????????<result?column="cartype"?property="type"/>??
6. ????????<association?property="engine"?resultMap="engineResult"/>??
7. ????????<association?property="brakes"?resultMap="brakesResult"/>??
8. ????</resultMap>??
9. ????<resultMap?type="org.apache.ibatis.submitted.associationtest.Engine"?id="engineResult">??
10. ????????<result?column="enginetype"?property="type"/>??
11. ????????<result?column="enginecylinders"?property="cylinders"/>??
12. ????</resultMap>??
13. ????<resultMap?type="org.apache.ibatis.submitted.associationtest.Brakes"?id="brakesResult">??
14. ????????<result?column="brakesType"?property="type"/>??
15. ????</resultMap>??
16. ??
17. ????<select?id="getCars"?resultMap="carResult">??
18. ????select?*?from?cars??
19. ??</select>??
20. ??
21. ????<select?id="getCarsNonUnique"?resultMap="carResult">??
22. ????select?1?as?carid,?cartype,?enginetype,?enginecylinders,?brakestype?from?cars??
23. ??</select>??
24. ??
25. ????<select?id="getCars2"?resultMap="carResult">??
26. ????select?1?as?carid,?cartype,?enginetype,?enginecylinders,?brakestype?from?cars?where?carid?in?(1,2)??
27. ??</select>??
28. ??
29. </mapper>??

<mapper namespace="org.apache.ibatis.submitted.associationtest.Mapper">

    <resultMap type="org.apache.ibatis.submitted.associationtest.Car" id="carResult">
        <id column="carid" property="id"/>
        <result column="cartype" property="type"/>
        <association property="engine" resultMap="engineResult"/>
        <association property="brakes" resultMap="brakesResult"/>
    </resultMap>
    <resultMap type="org.apache.ibatis.submitted.associationtest.Engine" id="engineResult">
        <result column="enginetype" property="type"/>
        <result column="enginecylinders" property="cylinders"/>
    </resultMap>
    <resultMap type="org.apache.ibatis.submitted.associationtest.Brakes" id="brakesResult">
        <result column="brakesType" property="type"/>
    </resultMap>

    <select id="getCars" resultMap="carResult">
    select * from cars
  </select>

    <select id="getCarsNonUnique" resultMap="carResult">
    select 1 as carid, cartype, enginetype, enginecylinders, brakestype from cars
  </select>

    <select id="getCars2" resultMap="carResult">
    select 1 as carid, cartype, enginetype, enginecylinders, brakestype from cars where carid in (1,2)
  </select>

</mapper>

其中的一个测试用例：

[java] view plaincopyprint?

1. @Test??
2. ?public?void?shouldGetAllCars()?{??
3. ???SqlSession?sqlSession?=?sqlSessionFactory.openSession();??
4. ???try?{??
5. ?????Mapper?mapper?=?sqlSession.getMapper(Mapper.class);??
6. ?????List<Car>?cars?=?mapper.getCars();??
7. ?????Assert.assertEquals(4,?cars.size());??
8. ?????Assert.assertEquals("VW",?cars.get(0).getType());??
9. ?????Assert.assertNotNull(cars.get(0).getEngine());??
10. ?????Assert.assertNull(cars.get(0).getBrakes());??
11. ?????Assert.assertEquals("Opel",?cars.get(1).getType());??
12. ?????Assert.assertNull(cars.get(1).getEngine());??
13. ?????Assert.assertNotNull(cars.get(1).getBrakes());??
14. ???}?finally?{??
15. ?????sqlSession.close();??
16. ???}??
17. ?}??

 @Test
  public void shouldGetAllCars() {
    SqlSession sqlSession = sqlSessionFactory.openSession();
    try {
      Mapper mapper = sqlSession.getMapper(Mapper.class);
      List<Car> cars = mapper.getCars();
      Assert.assertEquals(4, cars.size());
      Assert.assertEquals("VW", cars.get(0).getType());
      Assert.assertNotNull(cars.get(0).getEngine());
      Assert.assertNull(cars.get(0).getBrakes());
      Assert.assertEquals("Opel", cars.get(1).getType());
      Assert.assertNull(cars.get(1).getEngine());
      Assert.assertNotNull(cars.get(1).getBrakes());
    } finally {
      sqlSession.close();
    }
  }

association是嵌套查询中最简单的一种情况，像上述例子中，一般我们都会用一个Car对面包含所有的属性，这里的例子使用了嵌套对象，使对像的结构更鲜明。不过一般情况下很少会拆分一个对象为多个，用的多的时候是多表查询的嵌套。

上面XML中的
carResult和engieResult,brakesResult都是分别定义，carResult引用了另外两个resultMap。
对于不需要重用嵌套对象的情况，还可以直接这么写，把上面的XML修改后：

[html] view plaincopyprint?

1. <resultMap?type="org.apache.ibatis.submitted.associationtest.Car"?id="carResult">??
2. ????<id?column="carid"?property="id"/>??
3. ????<result?column="cartype"?property="type"/>??
4. ????<association?property="engine"?javaType="org.apache.ibatis.submitted.associationtest.Engine">??
5. ????????<result?column="enginetype"?property="type"/>??
6. ????????<result?column="enginecylinders"?property="cylinders"/>??
7. ????</association>??
8. ????<association?property="brakes"?resultMap="brakesResult"/>??
9. </resultMap>??

    <resultMap type="org.apache.ibatis.submitted.associationtest.Car" id="carResult">
        <id column="carid" property="id"/>
        <result column="cartype" property="type"/>
        <association property="engine" javaType="org.apache.ibatis.submitted.associationtest.Engine">
            <result column="enginetype" property="type"/>
            <result column="enginecylinders" property="cylinders"/>
        </association>
        <association property="brakes" resultMap="brakesResult"/>
    </resultMap>

如果有association方面问题可以参考（或在此留言）：

http://mybatis.github.io/mybatis-3/zh/sqlmap-xml.html

本节源码请看官方git:

https://github.com/mybatis/mybatis-3/tree/master/src/test/java/org/apache/ibatis/submitted/associationtest