Hi all,
I've been using Hibernate for the past two and a half years with great results, but there's something that's been bugging me for a long time now: why I can't get Hibernate to avoid fetching unnecessary data via HQL. Let's say I have the following tables:
Country: ID (PK), Description
Maker: ID (PK), Description, Country ID (FK)
Disk: ID (PK), Description, Maker ID (FK)
Computer: ID (PK), Description, Disk ID (FK)
So, every table has a numeric ID as its primary key, and Maker, Disk and Computer all contain foreign keys.
Country is mapped as follows:
Code:
<hibernate-mapping>
<class name="testdb.Country" table="country">
<id column="country_id" name="id" unsaved-value="none">
<generator class="assigned"/>
</id>
<property column="country_description" name="description"/>
</class>
</hibernate-mapping>
Maker is mapped as follows:
Code:
<hibernate-mapping>
<class name="testdb.Maker" table="maker">
<id column="maker_id" name="id" unsaved-value="none">
<generator class="assigned"/>
</id>
<property column="maker_description" name="description"/>
<many-to-one column="country_id" fetch="join"
name="country" not-found="ignore"/>
</class>
</hibernate-mapping>
(The mappings for Disk and Computer are very similar to that of Maker).
Now, if I try to list() the Computer table with the code below:
Code:
List<Computer> computers = (List<Computer>) listRecordsViaHql(
"SELECT comp " +
"FROM Computer comp " +
" LEFT JOIN FETCH comp.disk ");
The following SQL query is generated by Hibernate:
Code:
select computer0_.computer_id as computer1_3_0_,
disk1_.disk_id as disk1_2_1_,
computer0_.computer_description as computer2_3_0_,
computer0_.disk_id as disk3_3_0_,
disk1_.disk_description as disk2_2_1_,
disk1_.maker_id as maker3_2_1_
from computer computer0_
left outer join disk disk1_ on computer0_.disk_id = disk1_.disk_id
Which is fine. However, Hibernate generates a few extra SQL queries to fetch the Maker for every Disk:
Code:
select maker0_.maker_id as maker1_1_1_,
maker0_.maker_description as maker2_1_1_,
maker0_.country_id as country3_1_1_,
country1_.country_id as country1_0_0_,
country1_.country_description as country2_0_0_
from maker maker0_
left outer join country country1_ on maker0_.country_id =
country1_.country_id
where maker0_.maker_id = ?
Which I don't want Hibernate to do, since I'm not calling computer.getDisk().getMaker() in any point of my code. Aside from using "LEFT JOIN FETCH comp.disk.maker" in my HQL queries or using the Criteria API with max_fetch_depth set to high values, what should I do in order to keep Hibernate from generating these extra queries? I've created my test case with Hibernate 3.3.2, HSQLDB 1.8.0.10 and JDK 1.6.14. Any and all help on this issue would be greatly appreciated. Thanks,
Roger
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