Architectural suggestion

nickvajberg · **Joined:** Tue Oct 21, 2003 8:15 am **Posts:** 186

A feature I miss from both Hibernate and, in fact, all OODBMS's is a "object graph selection" mechanism/language.

By this, I mean a way of specifying which parts of an object graph should be returned from the database. This has important value in terms of performance.

Following references outside the selected graph closure would happend in a lazy manner, while everything within the closure is preloaded.

For instance:

Person[] persons = session.select("from Person (include Addresses (include Streets)) where Addresses.Country = 'Australia'");

Or something like that...

Any thoughts about such a feature?

gavin · **Posted:** Thu Oct 23, 2003 4:40 pm

HQL "LEFT JOIN FETCH" construct.

nickvajberg · **Joined:** Tue Oct 21, 2003 8:15 am **Posts:** 186

Ah - very clever!

max · **Posted:** Fri Oct 24, 2003 7:20 am

...but be aware that HQL currently only support one (1) FETCH per HQL.....

...but please raise a JIRA for having more than 1 FETCH per HQL so we can see how much people really want it ;)

drfish · **Posted:** Fri Oct 24, 2003 4:35 pm

having more than one fetch per query would be VERY usefull.. for our uses we would mark EVERYTHING either lazy or proxy, then based on use case "fetch" the proper portions of the graph.

I would also like to see some variation on fetch profiles that could be used in conjuction with HQL. maby "named" profiles defined in the mapping files that could be used within the queries:

mapping:

Code:

<fetchprofile name="cat_kittens_and_mate" class="eg.Cat">
  <collection-fetch property="kittens"/>
  <property-fetch property="mate"/>
</fetchprofile>

query:

Code:

select cat  from eg.Cat cat where cat.something=value using fetch profile cat_kittens_and_mate

This would allow the system archetects to define what are acceptable fetch profiles and application developers would just use one of the predefined fetch profiles

gavin · **Posted:** Fri Oct 24, 2003 5:10 pm

Max mis-spoke. You can have as many FETCH joins as you like. But only one may be of a collection.

max · **Posted:** Sat Oct 25, 2003 5:51 am

ah the small gritty details - but it sure would be nice to express the fetch strategy explictly instead of via a bunch of "external" Hibernate.initialize() calls ;)

gavin · **Posted:** Sat Oct 25, 2003 6:34 am

It would not be at all difficult and might indeed be quite nice to add fetch profiles to the Criteria API. eg.

Code:

session.createCriteria(Foo.class)
    .setFetchProfile("Foo.default")
    .add( Expression.isNotNull("bar") )
    .list();

the "fetch profile" would just be a named list of property names, and setFetchProfile() would be equivalent to calling setFetchMode("name", FetchMode.EAGER) for each.

This should be easy for whoever wants to implement it....