Nhibernate count distinct (based on multiple columns)

Question

Basically, i have been trying to do this (count distinct based on two columns):

select count(distinct(checksum(TableA.PropertyA, TableB.PropertyB))) 
from Ta         


        

Answer 1

Okay this is going to take a few steps, so bear with me. I'm assuming SQL server here, but the instructions should work for any dialect that supports checksum¹:

1. Create a custom dialect that supports the checksum function:

   public class MyCustomDialect : MsSql2008Dialect
   {
       public MyCustomDialect()
       {
           RegisterFunction("checksum", new SQLFunctionTemplate(NHibernateUtil.Int32, "checksum(?1, ?2)"));
       }
   }
   

2. Update your configuration to use the custom dialect (you can do this either in your configuration XML file or with code. See this answer for more information). Here's how I did it inside of my existing configuration code:

   configuration
       .Configure(@"hibernate.cfg.xml")
       .DataBaseIntegration(
           db => db.Dialect<MyCustomDialect>());
   

3. Create a custom projection that calls checksum. This step is optional-- you can call Projections.SqlFunction directly if you'd like, but I think refactoring it into a separate function is cleaner:

   public static class MyProjections 
   {
       public static IProjection Checksum(params IProjection[] projections)
       {
           return Projections.SqlFunction("checksum", NHibernateUtil.Int32, projections);   
       }
   }
   

4. Write your QueryOver query and call the custom projection:

   int count = session.QueryOver<TableA>(() => tableAAlias)
       .Where(p => p.PropertyA.IsLike("%123%"))
       .Left.JoinQueryOver(p => p.TableB, () => tableBAlias)
       .Select(
           Projections.Count(
               Projections.Distinct(
               MyProjections.Checksum(
                   Projections.Property(() => tableAAlias.PropertyA),
                   Projections.Property(() => tableBAlias.PropertyB)))))
       .SingleOrDefault<int>();
   

   This should generate SQL that looks like what you're after:

   SELECT count(distinct checksum(this_.PropertyA, tableba1_.PropertyB)) as y0_
   FROM   [TableA] this_
       left outer join [TableB] tableba1_
       on this_.TableBId = tableba1_.Id
   WHERE  this_.PropertyA like '%123%' /* @p0 */
   

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¹Still trying to figure out if there's a way to map a function without manually specifying the number of arguments