Question

I have a database schema that stores one “Page” with many “Revisions”. Like a simple wiki.

90% of the time when I load a page, I am just interested in the latest revision. However, sometimes I want all revisions.

With NHibernate I can map the Page to the Revisions, and tell it to lazy-load. However, when I access the latest revision, it will load all other revisions – a big waste of I/O.

My page class currently resembles:

public class Page
{
    public Page()
    {
        Revisions = new HashedSet<Revision>();
    }

    public virtual ISet<Revision> Revisions { get; private set; }

    public virtual Revision LatestRevision
    {
        get { return Revisions.OrderByDescending(x => x.Revised).FirstOrDefault(); }
    }

    public virtual Revision Revise()
    {
        var revision = new Revision();
        // ...
        revision.Entry = this;
        revision.Revised = DateTime.UtcNow;
        Revisions.Add(revision);
        return revision;
    }
}

How would I model this such that the LatestRevision is automatically loaded when the Page is loaded, but the other revisions are lazy-loaded if, for instance, I attempted to iterate them?

I would particularly like a solution that works with LINQ to NHibernate, but using ICriteria (or even SQL if I have to) is good enough.

Answer 1

I ended up going with the solution from here:

Partially Populate Child Collection with NHibernate

My page now has these properties:

public virtual Revision CurrentRevision 
{ 
    get
    {
        return _revision ?? Revisions.OrderByDescending(x => x.Revised).FirstOrDefault();
    }
    set { _revision = value; }
}

public virtual ISet<Revision> Revisions { get; private set; }

The loading code is:

public Page GetPage(string name)
{
    var entryHash = (Hashtable)_session.CreateCriteria<Page>("page")
        .Add(Restrictions.Eq("page.Name", name))
        .CreateCriteria("Revisions", "rev")
            .AddOrder(Order.Desc("rev.Revised"))
        .SetMaxResults(1)
        .SetResultTransformer(Transformers.AliasToEntityMap)
        .UniqueResult();
    var page = (Page)entryHash["page"];
    page.LatestRevision = (Revision)entryHash["rev"];
    return page;
}

NHProf shows this as the only query being executed now, which is perfect:

SELECT   top 1 this_.Id            as Id3_1_,
               this_.Name          as Name3_1_,
               this_.Title         as Title3_1_,
               rev1_.Id            as Id0_0_,
               rev1_.Body          as Body0_0_,
               rev1_.Revised       as Revised0_0_,
               ....
FROM     [Page] this_
         inner join [Revision] rev1_
           on this_.Id = rev1_.PageId
WHERE    this_.Name = 'foo' /* @p0 */
ORDER BY rev1_.Revised desc