What is the .Fetch.Select() in Fluent nHibernate?

Question

While developing with Fluent nHibernate, I notice that on relationships I can specify a Fetch property, with possible options of Select(), Join(), and Subselect().

I did some searches for these and yielded very little information. I did find them in the nHibernate documentation and the fluent nHibernate documentation, but it does little other than give their signatures, which doesn't help me too much.

I was wondering if there is any real explanation for what these are, and what they really do. I've been rather perplexed myself. From my own evaluation they seem to change the way that referenced entities are pulled into the object graph, but I've yet to entirely discern how they change it, and which one is optimal for what situation...

I did find this blog post (http://www.mkyong.com/hibernate/hibernate-fetching-strategies-examples/) that has a little bit of detail but I'm still pretty perplexed about the entire situation. I've also seen other examples that state using Select() is more optimal, but the reasoning behind it. Additionally I found a post at (http://community.jboss.org/wiki/AShortPrimerOnFetchingStrategies) that is geared towards the original Java Hibernate platform, but I presume the concept is the same. In this one, my theory seems to be blown a bit as it focuses more on the lazy loading aspect of what they do, but I've still not seen any really flat examples.

Answer 1

• Join fetching - NHibernate retrieves the associated instance or collection in the same SELECT, using an OUTER JOIN.

• Select fetching - a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you actually access the association.

• Subselect fetching - a second SELECT is used to retrieve the associated collections for all entities retrieved in a previous query or fetch. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you actually access the association.

Check out the fetching strategy document @ The Nhibernate Documentation

Answer 2

I'm not really familiar with nHibernate (I work with Hibernate and Java), but based on analogy, this enables you to specify association/collection property which you want to load eagerly, with the given entity. This is useful when you don't have full control over (n)Hibernate sessions (i.e if some other framework like Spring in Java is taking care of sessions/transactions). So your assumption is basically correct. Select, Join, and Subselect are the ways to obtain the related property, and determine what kind of query will be performed in database. Which one is optimal, really depends on the situation you have.

Hope this helps a little,
Cheers.