Hi

I have trouble with understanding when NHibernate uses outer-joins. According to the docs, when I have "hibernate.use_outer_join" set to true and "hibernate.max_fetch_depth" >=1 , all many-to-one relations should be fetched using the joins - or I am misunderstanding something.
http://www.hibernate.org/hib_docs/reference/en/html/session-configuration.html#configuration-optional-outerjoin

Actually I noticed no difference in generated queries, with different configuration values ( always a bunch of selects, number of objects X number of relations).
The only way when NHibernate issued single query with join is using "left join fetch object.relation" in the the HQL query.

Can anyone tell me how to force "outer joins" without explicitly specifing them in all queries?

Ok, after some investigation I see it's a stupid question.

I've read in the Hibernate FAQ: The session is a transaction-level cache of persistent objects. So if I load an object in the first query, then I don't need any join (and any futher selects for relations) when fetching object graph, e.g. when I do "select from customers" and then "select from orders" ( many orders -> 1 customer) within one session, NHibernate need only one "flat" select to load the orders. I can also force using "outer join" with "fetch" HQL keyword in the orders query: "select from orders o left join fetch o.customer".

IMHO it could be great if NH would use "join" or "flat query" depending on that the related object is already in the session or not, but I understand that it is hard (or impossible) to implement.

And the question:
Only one thing is still unclear for me - what is fetch=join and outer-join=true purpose ? It appears useless for me according to the above.

outer-join affects only Get and Load, and possibly criteria queries, as far as I understand. In HQL you need to write all the joins explicitly.

Ok, I understand , I will check it later. Thanks for the answer.