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         


        

Answer 1

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.

Answer 2

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())
    } 

Answer 3

Apparently there may be a solution as indicated in by using a combination of groupValBy and AnonymousObject<_,_> from the Linq RuntimeHelpers.

ex.

query {
    for d in context.table do
    let key = AnonymousObject<_,_>(d.col1,d.col2)
    groupValBy d key into g
    select (g.Key)
    } 

Answer 4

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

Answer 5

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 6

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())
}