groupby multiple columns in a F# 3.0 query

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.

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())
}

• groupValBy is described as "Selects a value for each element selected so far and groups the elements by the given key." The key is this case is still just based on student.Name - Jizugu
• So, why is there that student.Age? I can't believe there is no easy way to group by multiple columns in FSharp. Similar case I see now in join - how to write composite key? - tomasK
• Actually this will group only by student.Age. The first arg groupValBy is the value which will be stored, the second is the key to group by. Imagine the word by between the first and second args. - latkin

When using groupBy you need to select an aggregating function (e.g. count, sum, avg...).

• The example is really just for illustration, using select (g.Key) isn't useful but should work... "select (g.Count())" gives the same error. - Jizugu

First you have to remember that a query is translated into actual SQL at some point. It appears that linq does not support the use of multiple group keys as a Tuple<>. Therefore any transformation into Tuple<> has to be done after the database call has completed.

Second, you should be able to achieve multiple key grouping by performing multiple groupings behind each other on the respective keys:

query {
    for d1 in context.table do
    groupBy d1.col1 into g1
    for d2 in g1 do
    groupBy d2.col2 into g2
    select g2
}

Please have mercy with me if the syntax is not 100% since F# is not my native tongue :) The concept however should work just fine.

• Interestingly enough still get "Only parameterless constructors and initializers are supported in LINQ to Entities." - Jizugu

open Microsoft.FSharp.Linq.RuntimeHelpers
open System.Linq

query {
    for d in context.table do
    let t = MutableTuple<_,_>(Item1=d.col1,Item2=d.col2)
    groupValBy d t into g
    select (g.Key,g.Count())
    } 

• Seems to work. However having trouble getting anything beyond select (g.Key). select (g.Count()) gives an error and I'm not sure how to go about doing aggregation on other columns. Still playing with the idea though - Jizugu

//in F# 3.0
open Microsoft.FSharp.Linq.RuntimeHelpers
open Microsoft.FSharp.Linq.RuntimeHelpers.LeafExpressionConverter
open System.Linq

//[<CLIMutable>]
//type MyRecord = { Column1 : int; Column2 : int }
// require constructor in F#
// groupBy is not valid

type T(column1 : int, column2 : int)
    member val Colum1=colum1 with get,set
    membre val Colum2=colum2 with get,set

query {
    for d in context.table do
    groupValBy d (NewAnonymousObjectHelper(T(d.Colum1,d.Colume2))) into g
    select (g.Key)
    }

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

• Please add comments to help the asker understand. Otherwise this is "magic". - Eric Fossum
• new {d.col1, d.col2} constructs a C# anonymous type; this is not valid F#. - Jay