Are there any arithmetic operation projections in NHibernate?

问题

I would like to get this SQL from NHibernate:

SELECT SUM(color_pages) * SUM(total_pages)
FROM connector_log_entry
GROUP BY department_name

But I can't find any arithmetic operation (*) projections anywhere.

This is the code that I have so far:

Session.QueryOver<ConnectorLogEntry>()
       .SelectList(list => list
           .SelectGroup(m => m.DepartmentName)
           .WithAlias(() => dto.Department)
           .Select(Projections.Sum<ConnectorLogEntry>(m => m.TotalPages))
           //.Select(Projections.Sum<ConnectorLogEntry>(m => m.ColorPages))
           .WithAlias(() => dto.TotalColorPercentage))
       .TransformUsing(Transformers.AliasToBean<DepartmentConsumption>());

回答1:

Arithmetic operators can be used in criteria queries via the VarArgsSQLFunction SQL function. In your particular case, this would look something like:

Session.QueryOver<ConnectorLogEntry>()
    .SelectList(list =>
        list.SelectGroup(m => m.DepartmentName)
            .WithAlias(() => dto.Department)
            .Select(Projections.SqlFunction(
                new VarArgsSQLFunction("(", "*", ")"),
                NHibernateUtil.Int32,
                Projections.Sum<ConnectorLogEntry>(m => m.TotalPages),
                Projections.Sum<ConnectorLogEntry>(m => m.ColorPages)))
            .WithAlias(() => dto.TotalColorPercentage))
    .TransformUsing(Transformers.AliasToBean<DepartmentConsumption>());

This technique injects strings directly into the generated SQL, so you'll need to make sure the underlying database supports the operators you use.

回答2:

It's trivial with LINQ or HQL, but Criteria and QueryOver are not optimized for that (you have to use a SQL Projection)

HQL is almost the same as SQL:

select sum(ColorPages) * sum(TotalPages)
from ConnectorLogEntry
group by DepartmentName

LINQ is not hard either:

from entry in Session.Query<ConnectorLogEntry>()
group entry by entry.DepartmentName into g
select g.Sum(e => e.ColorPages) * g.Sum(e => e.TotalPages)

来源：https://stackoverflow.com/questions/4828552/are-there-any-arithmetic-operation-projections-in-nhibernate