Adding a projection to an NHibernate criteria stops it from performing default entity selection

Question

I'm writing an NHibernate criteria that selects data supporting paging. I'm using the COUNT(*) OVER() expression from SQL Server 2005(+) to get hold of the total number of available rows, as suggested by Ayende Rahien. I need that number to be able to calculate how many pages there are in total. The beauty of this solution is that I don't need to execute a second query to get hold of the row count.

However, I can't seem to manage to write a working criteria (Ayende only provides an HQL query).

Here's an SQL query that shows what I want and it works just fine. Note that I intentionally left out the actual paging logic to focus on the problem:

SELECT Items.*, COUNT(*) OVER() AS rowcount
FROM Items

Here's the HQL:

select
    item, rowcount()
from 
    Item item

Note that the rowcount() function is registered in a custom NHibernate dialect and resolves to COUNT(*) OVER() in SQL.

A requirement is that the query is expressed using a criteria. Unfortunately, I don't know how to get it right:

var query = Session
    .CreateCriteria<Item>("item")
    .SetProjection(
       Projections.SqlFunction("rowcount", NHibernateUtil.Int32));

Whenever I add a projection, NHibernate doesn't select item (like it would without a projection), just the rowcount() while I really need both. Also, I can't seem to project item as a whole, only it's properties and I really don't want to list all of them.

I hope someone has a solution to this. Thanks anyway.

Answer 1

I think it is not possible in Criteria, it has some limits.

You could get the id and load items in a subsequent query:

var query = Session
    .CreateCriteria<Item>("item")
    .SetProjection(Projections.ProjectionList()
       .Add(Projections.SqlFunction("rowcount", NHibernateUtil.Int32))
       .Add(Projections.Id()));

If you don't like it, use HQL, you can set the maximal number of results there too:

IList<Item> result = Session
    .CreateQuery("select item, rowcount() from item where ..." )
    .SetMaxResult(100)
    .List<Item>();

Answer 2

Use CreateMultiCriteria.

You can execute 2 simple statements with only one hit to the DB that way.

Answer 3

I am wondering why using Criteria is a requirement. Can't you use session.CreateSQLQuery? If you really must do it in one query, I would have suggested pulling back the Item objects and the count, like:

select {item.*}, count(*) over() 
from Item {item}

...this way you can get back Item objects from your query, along with the count. If you experience a problem with Hibernate's caching, you can also configure the query spaces (entity/table caches) associated with a native query so that stale query cache entries will be cleared automatically.

Answer 4

If I understand your question properly, I have a solution. I struggled quite a bit with this same problem.

Let me quickly describe the problem I had, to make sure we're on the same page. My problem came down to paging. I want to display 10 records in the UI, but I also want to know the total number of records that matched the filter criteria. I wanted to accomplish this using the NH criteria API, but when adding a projection for row count, my query no longer worked, and I wouldn't get any results (I don't remember the specific error, but it sounds like what you're getting).

Here's my solution (copy & paste from my current production code). Note that "SessionError" is the name of the business entity I'm retrieving paged data for, according to 3 filter criterion: IsDev, IsRead, and IsResolved.

ICriteria crit = CurrentSession.CreateCriteria(typeof (SessionError))
    .Add(Restrictions.Eq("WebApp", this));

if (isDev.HasValue)
    crit.Add(Restrictions.Eq("IsDev", isDev.Value));

if (isRead.HasValue)
    crit.Add(Restrictions.Eq("IsRead", isRead.Value));

if (isResolved.HasValue)
    crit.Add(Restrictions.Eq("IsResolved", isResolved.Value));

// Order by most recent
crit.AddOrder(Order.Desc("DateCreated"));

// Copy the ICriteria query to get a row count as well
ICriteria critCount = CriteriaTransformer.Clone(crit)
    .SetProjection(Projections.RowCountInt64());
critCount.Orders.Clear();

// NOW add the paging vars to the original query
crit = crit
    .SetMaxResults(pageSize)
    .SetFirstResult(pageNum_oneBased * pageSize);

// Set up a multi criteria to get your data in a single trip to the database
IMultiCriteria multCrit = CurrentSession.CreateMultiCriteria()
    .Add(crit)
    .Add(critCount);

// Get the results
IList results = multCrit.List();

List<SessionError> sessionErrors = new List<SessionError>();
foreach (SessionError sessErr in ((IList)results[0]))
    sessionErrors.Add(sessErr);

numResults = (long)((IList)results[1])[0];

So I create my base criteria, with optional restrictions. Then I CLONE it, and add a row count projection to the CLONED criteria. Note that I clone it before I add the paging restrictions. Then I set up an IMultiCriteria to contain the original and cloned ICriteria objects, and use the IMultiCriteria to execute both of them. Now I have my paged data from the original ICriteria (and I only dragged the data I need across the wire), and also a raw count of how many actual records matched my criteria (useful for display or creating paging links, or whatever). This strategy has worked well for me. I hope this is helpful.

Answer 5

I would suggest investigating custom result transformer by calling SetResultTransformer() on your session.

Answer 6

Create a formula property in the class mapping:

<property name="TotalRecords" formula="count(*) over()" type="Int32" not-null="true"/>;

IList<...> result = criteria.SetFirstResult(skip).SetMaxResults(take).List<...>();
totalRecords = (result != null && result.Count > 0) ? result[0].TotalRecords : 0;
return result;