How to Eager Load Associations without duplication

Question

I'd need to load a list of very large objects with so many children and children of children. what's the best approach to take?

I'm using Oracle 11g database and I've written the below method but it results in cartesian product (duplicated results):

 public IList<ARNomination> GetByEventId(long eventId)
        {
            var session = this._sessionFactory.Session;

            var nominationQuery = session.Query<ARNomination>().Where(n => n.Event.Id == eventId);

            using (var trans = session.Transaction)
            {
                trans.Begin();

                // this will load the Contacts in one statement
                nominationQuery
                    .FetchMany(n => n.Contacts)
                    .ToFuture();

                // this will load the CustomAttributes in one statement
                nominationQuery
                    .FetchMany(n => n.CustomAttributes)
                    .ToFuture();

                // this will load the nominations but joins those two tables in one statement which results in cartesian product
                nominationQuery
                    .FetchMany(n => n.CustomAttributes)
                    .FetchMany(n => n.Contacts)
                    .ToFuture();

                trans.Commit();
            }

            return nominationQuery.ToList();
        }

Answer 1

I concur with @RadimKöhler as soon as you eager load more than one collection then a Cartesian product always occurs. For selecting a suitable batch size then I would probably choose this to be the same as the page size as it just feels right... (no evidence why though)

There is another technique that you may feel is a better fit and that is to read this blog post by Ayende which shows you how you can send two future queries at the same time to eager load multiple collections that soul job is to load each collection singly.

However whichever route you take I suggest throwing a profiler at the results to see which performs better for you...

Answer 2

Fetching Collections is a difficult operation. It has many side effects (as you realized, when there are fetched more collections). But even with fetching one collection, we are loading many duplicated rows.

In general, for collections loading, I would suggest to use the batch processing. This will execute more SQL queries... but not so much, and what is more important, you can do paging on the root list ARNomination.

See: 19.1.5. Using batch fetching you can find more details.

You have to mark your collections and/or entities with an attribute batch-szie="25".

xml:

<bag name="Contacts" ... batch-size="25">
...

fluent:

HasMany(x => x.Contacts)
  ...
  .BatchSize(25)

Please, check few arguments here:

• NHibernate QueryOver with Fetch resulting multiple sql queries and db hits
• Is this the right way to eager load child collections in NHibernate
• https://stackoverflow.com/q/18419988/1679310