groupby multiple columns in a F# 3.0 query

Question

Just trying out F# 3.0 and hit a bit of a wall when it comes to grouping by multiple columns. The obvious thing to try was

query {     for d in context.table do     groupBy (d.col1,d.col2) into g     select (g.Key) } 

But I get a "Only parameterless constructors and initializers are supported in LINQ to Entities." exception.

I can't seem to find an example on msdn

http://msdn.microsoft.com/en-us/library/hh225374(v=vs.110).aspx

http://msdn.microsoft.com/en-us/library/hh361035(v=vs.110).aspx

And I realize my question is similar to " Entity Framework and Anonymous Types in F#" but it seems to be powerpack/F#2.x focused and I'm hoping F# 3.0 has an elegant answer... Any ideas?

UPDATE:

I came across the CLIMutable attribute from reading Brian's post at:

http://blogs.msdn.com/b/fsharpteam/archive/2012/07/19/more-about-fsharp-3.0-language-features.aspx

I was pretty optimistic so I tried

[<CLIMutable>] type MyRecord = { Column1 : int; Column2 : int }  query {     for d in context.table do     groupBy {Column1 = col1; Column2 = col2} into g     select (g.Key) } 

Unfortunately I get the exact same exception.

Answer 1

The following is an example of multiple columns being used for grouping in c# and converted to f# (overly paranoid management has made me rename everything, but I believe I have been consistent):

(TheDatabase was generated by SqlMetal, GetSummedValuesResult is a F# record type)

c#

public static class Reports {     public static IReadOnlyList<GetSummedValuesResult> GetSummedValues(TheDatabase db, DateTime startDate, DateTime? endDate)     {         var query =             from sv in db.SomeValues              where (sv.ADate >= startDate && sv.ADate <= (endDate ?? startDate))              group sv by new { sv.ADate, sv.Owner.Name } into grouping              select new GetSummedValuesResult(                 grouping.Key.ADate,                 grouping.Key.Name,                 grouping.Sum(g => g.Value)             );          return query.ToList();     } } 

f#

type Reports() =     static member GetSummedValues (db:TheDatabase) startDate (endDate:Nullable<DateTime>) =         let endDate = if endDate.HasValue then endDate.Value else startDate          let q = query {             for sv in db.SomeValues do             where (sv.ADate >= startDate && sv.ADate <= endDate)              let key = AnonymousObject<_,_>(sv.ADate, sv.Owner.Name)             groupValBy sv key into grouping              select {                 ADate        = grouping.Key.Item1;                 AName        = grouping.Key.Item2;                 SummedValues = grouping.Sum (fun (g:TheDatabaseSchema.SomeValues) -> g.Value)             }         }          List(q) :> IReadOnlyList<GetSummedValuesResult> 

So the thing to use is Microsoft.FSharp.Linq.RuntimeHelpers.AnonymousObject

Note that you should not use the Seq module for aggregation functions!!

SummedValues  = grouping |> Seq.sumBy (fun g -> g.SomeValues) 

Although this WILL WORK, it does the aggregation on the client side, rather than formulating appropriate SQL.

Answer 2

I see this in first of your links, I think it is what you want:

query {     for student in db.Student do     groupValBy student.Name student.Age into g     select (g, g.Key, g.Count()) }