Mapper的自动映射(Auto-mapping)

正如你在前面一节看到的，在简单的场景下，MyBatis可以替你自动映射查询结果。如果遇到复杂的场景，你需要构建一个result map。但是在本节你将看到，你也可以混合使用这两种策略。让我们到深一点的层面上看看自动映射是怎样工作的。

当自动映射查询结果时，MyBatis会获取sql返回的列名并在java类中查找相同名字的属性（忽略大小写）。这意味着如果Mybatis发现了ID列和id属性，Mybatis会将ID的值赋给id。

通常数据库列使用大写单词命名，单词间用下划线分隔；而java属性一般遵循驼峰命名法。为了在这两种命名方式之间启用自动映射，需要将 mapUnderscoreToCamelCase设置为true。

自动映射甚至在特定的result map下也能工作。在这种情况下，对于每一个result map,所有的ResultSet提供的列，如果没有被手工映射，则将被自动映射。自动映射处理完毕后手工映射才会被处理。在接下来的例子中， id 和 userName列将被自动映射， hashed_password 列将根据配置映射。

<select id="selectUsers" resultMap="userResultMap">
  select
    user_id   as "id",
    user_name  as "userName",
    hashed_password
  from some_table
  where id = #{id}
</select>

<resultMap id="userResultMap" type="User">
  <result property="password" column="hashed_password"/>
</resultMap>

There are three auto-mapping levels:

NONE - disables auto-mapping. Only manually mapped properties will be set.
PARTIAL - will auto-map results except those that have nested result mappings defined inside (joins).
FULL - auto-maps everything.

The default value is PARTIAL, and it is so for a reason. When FULL is used auto-mapping will be performed when processing join results and joins retrieve data of several different entities in the same row hence this may result in undesired mappings. To understand the risk have a look at the following sample:

<select id="selectBlog" resultMap="blogResult">
  select
    B.id,
    B.title,
    A.username,
  from Blog B left outer join Author A on B.author_id = A.id
  where B.id = #{id}
</select>

<resultMap id="blogResult" type="Blog">
  <association property="author" resultMap="authorResult"/>
</resultMap>
<resultMap id="authorResult" type="Author">
  <result property="username" column="author_username"/>
</resultMap>

With this result map both Blog and Author will be auto-mapped. But note that Author has an id property and there is a column named id in the ResultSet so Author's id will be filled with Blog's id, and that is not what you were expecting. So use the FULL option with caution.

Regardless of the auto-mapping level configured you can enable or disable the automapping for an specific ResultMap by adding the attribute autoMapping to it:

<resultMap id="userResultMap" type="User" autoMapping="false">
    <result property="password" column="hashed_password"/>
</resultMap>

20:Mapper的自动映射(Auto-mapping)

Mapper的自动映射(Auto-mapping)