NHibernate select complex sum - nhibernate

Question

I have an entity:
class Entity
{
public int A { get; set; }
public int B { get; set; }
public int C { get; set; }
}
I want to select sum of (A-B-C). So I want to run sql like this:
SELECT SUM(A-B-C) FROM Entity
I can achieve it by SqlProjection:
QueryOver.Of<Entity>().Select(Projections.SqlProjection("SUM(A-B-C) AS total", new[] { "total" }, new IType[] { NHibernateUtil.Int32 }));
But I do not want to use strings. How it can be done in other way?

Answer 1

Unfortunately NHibernate doesn't have built in arithmetic operators. Piggybacking on this question and answer, here are a few options:
Use VarArgsSQLFunction directly:
var subtractFunction = new VarArgsSQLFunction(string.Empty, " - ", string.Empty);
session.QueryOver<Entity>(() => entityAlias)
.Select(
Projections.Sum(
Projections.SqlFunction(
subtractFunction, NHibernateUtil.Int32,
Projections.Property(() => entityAlias.A),
Projections.Property(() => entityAlias.B),
Projections.Property(() => entityAlias.C)
)
)
)
.SingleOrDefault<int?>()
This is the most straightforward way to accomplish this, but there are a few ways to dress it up.
Create your own dialect and register a - function:
public class MyDialect : MsSql2008Dialect
{
public MyDialect()
{
this.RegisterFunction("-", new VarArgsSQLFunction(string.Empty, " - ", string.Empty));
}
}
session.QueryOver<Entity>(() => entityAlias)
.Select(
Projections.Sum(
Projections.SqlFunction(
"-", NHibernateUtil.Int32,
Projections.Property(() => entityAlias.A),
Projections.Property(() => entityAlias.B),
Projections.Property(() => entityAlias.C)
)
)
)
.SingleOrDefault<int?>()
This basically allows you to avoid redefining the - function every time you use it, and is a bit cleaner.
You can go even further and refactor the projection into an extension method:
public static class CustomProjections
{
public static IProjection Subtract(IType type, params IProjection[] projections)
{
return Projections.SqlFunction("-", type, projections);
}
}
session.QueryOver<Entity>(() => entityAlias)
.Select(
Projections.Sum(
CustomProjections.Subtract(
NHibernateUtil.Int32,
Projections.Property(() => entityAlias.A),
Projections.Property(() => entityAlias.B),
Projections.Property(() => entityAlias.C)
)
)
)
.SingleOrDefault<int?>()
All of these generate the following SQL:
SELECT
sum(this_.A - this_.B - this_.C) as y0_
FROM
Entity this_