How hibernate load object from DB?

fcaponi · **Posted:** Fri Feb 20, 2004 10:35 am

Hi,
I'm new using Hibernate 2.1.2, and I'm experiencing a performance problem.

I have two tables, Address and City, and they have a relationship "many-to-one" in the Address mapping file:

Code:

<many-to-one
        name="city"  class="com.test.hibernate.City"
        not-null="true">
        <column name="ID_CITY" />
</many-to-one>

When I retrieve the complete list of the Address object with the command:

Code:

hibSession.find("select add from Address add where add.id < 300 ")

Looking to the log i have seen that hibernate execute an initial query with the Select command on the table Address and then execute another query for each distinct City.

If I modify the query command in

Code:

hibSession.find("select add, add.city from Address add where add.id < 300 ")

it load the same object of type ind, but execute only a query with the correct join operator (inner or outer based on mapping).

Why this?

Is a behaviour by design or is a bug?

I would like that each object was loaded with a single SQL query... Can i control this with configuration parameter?

TIA,
Franco

michael · **Posted:** Fri Feb 20, 2004 10:39 am

You have full control over how Hibernate loads objects. You can use lazy=true for your associations to only load the collection contents when accessing the collections.

You can also use "from Address add join fetch add.city" to load the associated citys by joins in the query.

fcaponi · **Posted:** Fri Feb 20, 2004 10:49 am

My reference say that the tag "many-to-one" did not provide an attribute "lazy=true"!

I would like to access the Address object without listing in the query all other objects in the mapping file to workaround the problem.

Really I have experienced this problem with a chain more complex (six level of parent child).

michael · **Posted:** Fri Feb 20, 2004 10:56 am

Take a look at http://www.hibernate.org/162.html, basically if you want lazy loading on the one-side, you have to use proxys. You can also use outer-join=true in the mapping to allways do outer-join fetching. Set hibernate.max_fetch_depth to a positive value if you do.

Ricardo · **Posted:** Fri Feb 20, 2004 10:58 am

fcaponi wrote:

My reference say that the tag "many-to-one" did not provide an attribute "lazy=true"!

you must define the associate class to use a proxy.

fcaponi · **Posted:** Fri Feb 20, 2004 11:19 am

Thanks to all!!!

With this parameter i have obtained that hibernate don't load any referenced table until i need them... but can i have problems if i set the proxy attribute on each mapping?

fcaponi · **Posted:** Mon Feb 23, 2004 9:23 am

I'm experiencing problems with proxies because when i access list from my views, session is already closed.

Reading chapter 3.5.2. Outer Join Fetching i have seen that what i need can be configured with:

Code:

<property name="hibernate.use_outer_join">true</property>
<property name="hibernate.max_fetch_depth">3</property>

and on the many-to-one relationship the parameter outer-join has been forced to "true" (also if the auto value can be sufficient) but it don't work.

From log i have seen that outer join whas used on other command after first... It's correct or is a bug?

Any Ideas?

emmanuel · **Posted:** Tue Feb 24, 2004 4:57 pm

Can you reformulate please.

using many-to-one auto with a proxied one side, won't load it, as per the doc.