Get “surrounding” rows in NHibernate query

Question

I am looking for a way to retrieve the "surrounding" rows in a NHibernate query given a primary key and a sort order?

E.g. I have a table with log entries and I want to display the entry with primary key 4242 and the previous 5 entries as well as the following 5 entries ordered by date (there is no direct relation between date and primary key). Such a query should return 11 rows in total (as long as we are not close to either end).

The log entry table can be huge and retrieving all to figure it out is not possible.

Is there such a concept as row number that can be used from within NHibernate? The underlying database is either going to be SQlite or Microsoft SQL Server.

Edited Added sample

Imagine data such as the following:

Id   Time
4237 10:00
4238 10:00
1236 10:01
1237 10:01
1238 10:02
4239 10:03
4240 10:04
4241 10:04
4242 10:04   <-- requested "center" row
4243 10:04
4244 10:05
4245 10:06
4246 10:07
4247 10:08

When requesting the entry with primary key 4242 we should get the rows 1237, 1238 and 4239 to 4247. The order is by Time, Id.

Is it possible to retrieve the entries in a single query (which obviously can include subqueries)? Time is a non-unique column so several entries have the same value and in this example is it not possible to change the resolution in a way that makes it unique!

Answer 1

"there is no direct relation between date and primary key" means, that the primary keys are not in a sequential order?

Then I would do it like this:

Item middleItem = Session.Get(id);

IList<Item> previousFiveItems = Session.CreateCriteria((typeof(Item))
  .Add(Expression.Le("Time", middleItem.Time))
  .AddOrder(Order.Desc("Time"))
  .SetMaxResults(5);

IList<Item> nextFiveItems = Session.CreateCriteria((typeof(Item))
  .Add(Expression.Gt("Time", middleItem.Time))
  .AddOrder(Order.Asc("Time"))
  .SetMaxResults(5);

There is the risk of having several items with the same time.

---

Edit

This should work now.

Item middleItem = Session.Get(id);

IList<Item> previousFiveItems = Session.CreateCriteria((typeof(Item))
  .Add(Expression.Le("Time", middleItem.Time)) // less or equal
  .Add(Expression.Not(Expression.IdEq(middleItem.id))) // but not the middle
  .AddOrder(Order.Desc("Time"))
  .SetMaxResults(5);

IList<Item> nextFiveItems = Session.CreateCriteria((typeof(Item))
  .Add(Expression.Gt("Time", middleItem.Time))  // greater 
  .AddOrder(Order.Asc("Time"))
  .SetMaxResults(5);

Answer 2

This should be relatively easy with NHibernate's Criteria API:

List<LogEntry> logEntries = session.CreateCriteria(typeof(LogEntry))
.Add(Expression.InG<int>(Projections.Property("Id"), listOfIds))
.AddOrder(Order.Desc("EntryDate"))
.List<LogEntry>();

Here your listOfIds is just a strongly typed list of integers representing the ids of the entries you want to retrieve (integers 4242-5 through 4242+5 ).

Of course you could also add Expressions that let you retrieve Ids greater than 4242-5 and smaller than 4242+5.

Answer 3

Stefan's solution definitely works but better way exists using a single select and nested Subqueries:

ICriteria crit = NHibernateSession.CreateCriteria(typeof(Item));

        DetachedCriteria dcMiddleTime =
            DetachedCriteria.For(typeof(Item)).SetProjection(Property.ForName("Time"))
            .Add(Restrictions.Eq("Id", id));

        DetachedCriteria dcAfterTime =
            DetachedCriteria.For(typeof(Item)).SetMaxResults(5).SetProjection(Property.ForName("Id"))
            .Add(Subqueries.PropertyGt("Time", dcMiddleTime));
        DetachedCriteria dcBeforeTime =
            DetachedCriteria.For(typeof(Item)).SetMaxResults(5).SetProjection(Property.ForName("Id"))
                .Add(Subqueries.PropertyLt("Time", dcMiddleTime));

        crit.AddOrder(Order.Asc("Time"));
        crit.Add(Restrictions.Eq("Id", id) || Subqueries.PropertyIn("Id", dcAfterTime) ||
                 Subqueries.PropertyIn("Id", dcBeforeTime));

        return crit.List<Item>();

This is NHibernate 2.0 syntax but the same holds true for earlier versions where instead of Restrictions you use Expression.

I have tested this on a test application and it works as advertised