Eagerly load recursive relation

Question

I have a recursive one-to-many relationship that has the default lazy value of true. What code can I write against the NH API that will efficiently retrieve the ENTIRE tree

Answer 1

See Ayende's site: Efficiently Selecting a Tree. I have successfully used this technique in my own applications. With ICriteria, it looks like this:

session.CreateCriteria<Category>()
    .SetFetchMode("SubCategories", FetchMode.Join)
    .SetResultTransformer(new DistinctRootEntityResultTransformer())
    .List<Category>()
    .Where(x => x.ParentCategory == null);

The main difference between this version and what you tried is how the "ParentCategory == null" filter is applied. It has to be left out of the query that is sent to the database in order to retrieve the whole tree - but we still need the query to only return the root nodes of the tree, so we'll use linq to find those after the database query has completed.

Answer 2

Not sure if it helps but take a look at : map a tree in NHibernate

Answer 3

I used Daniel's code as a bases for solving the problem. I also experimented with the equivalent HQL that I shared below. The HQL executed slightly faster, but I went with ICriteria since I could then choose between FetchModel.Join and FetchModel.Lazy.

session.CreateQuery( "from Category as c left join fetch c.SubCategories" )
  .SetResultTransformer( new DistinctRootEntityResultTransformer() )
  .List<Category>()
  .Where( c => c.ParentCategory == null );