Question

It’s a very simple mapping.

The database is mysql5.0

User <-one to many-> Blog

mappings:

<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>

I use the “fetch=join” in the <set> property,When I use the session.get() method to get the User instance like that

User user = (User)session.get(User.class, new Long(1));
    for (Blog blog : user.getBlogs()) {
        System.out.println(blog.getName());
    }

the output SQL is still “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=?

I try it in hibernate 3.2 and 3.6 ,the result is same ,is this a bug or something?

Answer 1

I think the fetch strategy will only be applied if the association is eagerly fetched. You didn’t mark the set with lazy="false", so the set is lazy-loaded the first time it’s accessed.

Note that setting lazy="false" will make Hibernate load the blogs of a user each time the user is loaded. You should perhaps let the association with lazy="true", and use an ad-hoc HQL query with join fetching when you need a user with his blogs:

select u from User u left join fetch u.blogs