Hello,

I want to use Hibernate with Parent / Child relationships and I have questions about it. I can't find answer in the userguide...

When someone makes a query, only elements are retrieved and what about the relations? A 'getParent' ( or equivalent ) make a new request or the instance is yet retrieved ( with a parameter to tell the deep of recursivity: for example tow levels )?

The aim of my question is to make the equivalent of a UNIQUE sql request that retrieve a list of people with informations about parents... And what about the performance if the equivalent in Hibernate make several requests?

Thanks by advance for your help,
Thierry

Hum, suchs question are answered in the reference guide, and are hard to sumarize.
The answer is 'it depends'.

I'll give you a start point.

On each collection relationship, you can add the lazy="true" parameter
if lazy=false, (and outer join capabilities = true), there will be 1 request
if lazy = true, no request on collectioned data are performed until you really access those data (ex: obj.getCollection.get(0)).

On object the behavior is done by the proxy attribute of class tag.

Now you can reread the doc, and make simple sample to see what hibernate do.

Ok, I will reread the docs... Then I make a hibernate config of a part of my model.

I see in the forum some other hints in the following thread:
http://forum.hibernate.org/viewtopic.ph ... rent+query

I think it's this kind of request I need in addition to the config you said.

Thanks for your help,
Thierry

The request made in this thread is quite strange.
"select a from a left outer join b" make an outer join for nothing because you asked for a and no particular clause is used on b attributes.

The proper request is "from a" (I think an outer join is performed by Hibernate in this case), but the user don't care.

The fetch keyword is useful when you explicitly set lazy=true or have a proxy on the children and want to populate parent and children in one request.

Hi,

I got the same question, but in a different particular case: i am using arrays.

I got a database with a high number of tables, and all relations are made by many-to-one or array collections. One of those tables ir a 'directory table', including the root directory that contains ALL the objects of ALL tables of the database. When I load the root directory, I'm load the intire daatabase!

How can I prevent that?

1) Using lazy="true" is not a solution, because I am using arrays.

2) Using outer-joins is not a solution because my queries all require diferente graph scopes.

3) using manual fetching is not a solution, because some of these tables (includind the 'directory table') are recursive, ie, they have foreign keys to the same table.

How can I solve this problem? Is there any other solutions beside these three?

Thanks in advance,
Joao Rangel

Use proxies?


Really, the problem is that you use arrays. Don't. Arrays are bad. The JVM gives me no way to proxy them.