Getting count with NHibernate + Linq + Future - linq

Question

I want to do paging with NHibernate when writing a Linq query. It's easy to do something like this:
return session.Query<Payment>()
.OrderByDescending(payment => payment.Created)
.Skip((page - 1)*pageSize)
.Take(pageSize)
.ToArray();
But with this I don't get any info about the total number of items. And if I just do a simple .Count(), that will generate a new call to the database.
I found this answer which solved it by using future. But it uses Criteria. How can I do this with Linq?

Answer 1

The difficulty with using Futures with LINQ is that operations like Count execute immediately.
As #vandalo found out, Count() after ToFuture() actually runs the Count in memory, which is bad.
The only way to get the count in a future LINQ query is to use GroupBy in an invariant field. A good choice would be something that is already part of your filters (like an "IsActive" property)
Here's an example assuming you have such a property in Payment:
//Create base query. Filters should be specified here.
var query = session.Query<Payment>().Where(x => x.IsActive == 1);
//Create a sorted, paged, future query,
//that will execute together with other statements
var futureResults = query.OrderByDescending(payment => payment.Created)
.Skip((page - 1) * pageSize)
.Take(pageSize)
.ToFuture();
//Create a Count future query based on the original one.
//The paged query will be sent to the server in the same roundtrip.
var futureCount = query.GroupBy(x => x.IsActive)
.Select(x => x.Count())
.ToFutureValue();
//Get the results.
var results = futureResults.ToArray();
var count = futureCount.Value;
Of course, the alternative is doing two roundtrips, which is not that bad anyway. You can still reuse the original IQueryable, which is useful when you want to do paging in a higher-level layer:
//Create base query. Filters should be specified here.
var query = session.Query<Payment>();
//Create a sorted, paged query,
var pagedQuery = query.OrderByDescending(payment => payment.Created)
.Skip((page - 1) * pageSize)
.Take(pageSize);
//Get the count from the original query
var count = query.Count();
//Get the results.
var results = pagedQuery.ToArray();
Update (2011-02-22): I wrote a blog post about this issue and a much better solution.

Answer 2

The following blog post has an implementation of ToFutureValue that works with LINQ.
http://sessionfactory.blogspot.com.br/2011/02/getting-row-count-with-future-linq.html
It has a small error on the following line that must be changed from this.
var provider = (NhQueryProvider)source.Provider;
To this:
var provider = (INhQueryProvider)source.Provider;
After apply the change you can use que queries in this way:
var query = session.Query<Foo>();
var futureCount = query.ToFutureValue(x => x.Count());
var page = query.Skip(pageIndex * pageSize).Take(pageSize).ToFuture();

Answer 3

var query = Session.QueryOver<Payment>()
.OrderByDescending(payment => payment.Created)
.Skip((page -1 ) * pageSize)
.Take(pageSize)
This is something I just discovered that the Linq to NH handles just fine, the ToRowCountQuery removes take/skip from the query and does a future row count.
var rowCount = query.ToRowCountQuery().FutureValue<int>();
var result = query.Future();
var asArray = result.ToArray();
var count = rowCount.Value();

Answer 4

Ok, it seems it should be working in your case, but I not tested:
return session.QueryOver<Payment>()
.Skip((page - 1) * pageSize)
.Take(pageSize)
.SelectList(r => r.SelectCount(f => f.Id))
.List<object[]>().First();
Test first before upvoting ;)
UPD: sorry, as I understand you now, you need to get Count of all items. Then you need to run the query without paging:
return session.QueryOver<Payment>()
.SelectList(r => r.SelectCount(f => f.Id))
.List<object[]>().First();