这是一个非常简单的映射。
mysql5.0
User <-一对多-> Blog
映射:
<class name="com.aaa.model.User" table="users">
<id name="id" column="ID">
<generator class="native"></generator>
</id>
<property name="username" column="USERNAME" type="string"/>
<property name="age" column="AGE" type="int"/>
<set name="blogs" inverse="true" fetch="join">
<key column="userid"/>
<one-to-many class="com.aaa.model.Blog"/>
</set>
</class>
<class name="com.aaa.model.Blog" table="blog">
<id name="id" column="ID">
<generator class="native"></generator>
</id>
<property name="name" column="NAME" type="string" length="50"/>
<property name="hits" column="HITS" type="integer" length="11"/>
</class>
我在<set>
属性中使用"fetch=join",当我使用session.get()方法来获取User实例时,
User user = (User)session.get(User.class, new Long(1));
for (Blog blog : user.getBlogs()) {
System.out.println(blog.getName());
}
输出SQL仍然是"N+1"
select
user0_.ID as ID0_0_,
user0_.USERNAME as USERNAME0_0_,
user0_.AGE as AGE0_0_
from
users user0_
where
user0_.ID=?
select
blogs0_.userid as userid0_1_,
blogs0_.ID as ID1_,
blogs0_.ID as ID1_0_,
blogs0_.NAME as NAME1_0_,
blogs0_.HITS as HITS1_0_
from
blog blogs0_
where
blogs0_.userid=?
我尝试它在hibernate 3.2和3.6,结果是一样的,这是一个bug还是什么?
我认为只有在急切获取关联时才会应用获取策略。您没有用lazy="false"
标记该集合,因此该集合在第一次访问时是惰性加载的。
注意,设置lazy="false"
将使Hibernate在每次加载用户时加载该用户的博客。您可能应该让该关联与lazy="true"
关联,并在需要用户的博客时使用具有连接抓取功能的临时HQL查询:
select u from User u left join fetch u.blogs