F# Query Expression using join & groupBy in same query

Question

I decided to learn how to use query expressions in F# and found the official Microsoft documentation. The caveats don't seem spelled out so I'm sure I've hit a problem with a simple fix but don't know why I'm getting the error it shows.

My thought was to write a query that did both a join and grouping in the same go. For example, using their sample 'MyDatabase' I thought I would try to find the number of classes that each student is enrolled in. After writing the following query, the compiler flags no warning, but when I go to run it it gives an error.

My query expression:

query {
    for student in db.Student do
    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)

    groupBy student into group

    select (group.Key, group.Count())

}

|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student.Name classCount)

The error:

System.InvalidOperationException: Could not format node 'New' for execution as SQL.
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitNew(SqlNew sox)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
>    at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitAlias(SqlAlias alias)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitSelect(SqlSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitScalarSubSelect(SqlSubSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitRow(SqlRow row)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitSelect(SqlSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.Format(SqlNode node, Boolean isDebug)
   at System.Data.Linq.SqlClient.SqlFormatter.Format(SqlNode node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(ResultShape resultShape, Type resultType, SqlNode node, ReadOnlyCollection`1 parentParameters, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at <StartupCode$FSI_0008>.$FSI_0008.main@() in C:\Users\JDKS\Library\query expressions.fsx:line 129
Stopped due to error

I even thought I could get out of this issue by using a subquery:

query {
    for student in db.Student do
    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)

    let count = query {
        for s in student.Name do
        select course.CourseID
        count
    }

    select (student.Name, count)

}

|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student classCount)

but that gave an even larger error:

> System.NotSupportedException: Sequence operators not supported for type 'System.String'.
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.ConvertToFetchedSequence(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSequence(SqlSelect sel)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitScalarSubSelect(SqlSubSelect ss)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSubSelect(SqlSubSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.FetchExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitNew(SqlNew sox)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitIncludeScope(SqlIncludeScope scope)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Bind(SqlNode node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(ResultShape resultShape, Type resultType, SqlNode node, ReadOnlyCollection`1 parentParameters, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at <StartupCode$FSI_0015>.$FSI_0015.main@() in C:\Users\JDKS\Library\query expressions.fsx:line 144
Stopped due to error

My guess is that I don't truly understand what's going on under the hood enough to work out the error or lead myself to a solution. Is it possible to query for what I want here? Is there a workaround?

I think it's because of the tuple at the end of the query. Try without it. — Fyodor Soikin, Jul 26 '17 at 01:37
I tried it without the tuple and it didn't work. I then tried grouping first and THEN joining and it succeeded. But it's not what I want. You can only `groupBy` `student.StudentID` because that's the column to join on to `CourseSelection`. I'm trying to work that back to name, but I just can't believe there isn't a way to group on name and count courses directly. — jks612, Jul 26 '17 at 03:12
The `for s in student.Name` is trying to iterate on the characters in the name, that's why you have this error in the second query — Pierre Irrmann, Jul 26 '17 at 13:43

score 1 · Accepted Answer · answered Jul 26 '17 at 14:28

1

When I tried running it locally I got this exception

System.Exception: Grouping over multiple tables is not supported yet

I am not sure why you would be getting the less pretty version of that error. But if you pull the GroupBy out of the query, things seem to work fine.

query {
    for student in db.Student do
    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)
    select (student.Name, course.Id)
} 
|> Seq.countBy snd
|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student classCount)

answered Jul 26 '17 at 14:28

JosephStevens

1,668
1
15
17

I think your countBy is supposed to be `fst` not `snd` – jks612 Jul 26 '17 at 14:41
That's really weird that my error is so large. Are you running F# version 14.0.23413.0 ? – jks612 Jul 26 '17 at 14:43
1

But this will do the grouping on the client. Huge performance penalty. – Fyodor Soikin Jul 26 '17 at 21:17

score 0 · Answer 2 · answered Jul 26 '17 at 13:45

0

I believe that the reason why your first query is not working is because of the groupBy student into group.

I think what you actually want to write is groupBy student.StudentID into group.

answered Jul 26 '17 at 13:45

Pierre Irrmann

227
1
6

jks612 · Answer 3 · 2017-08-10T19:59:21.523

I found an answer from a similar question:

query {
    for student in db.Student do

    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)

    let count = query { 
        for c in db.CourseSelection do
        where (c.StudentID = student.StudentID)
        select c
        count
    }


    select (student.Name, count)
    distinct

}

|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student classCount)

The underlying paradigm of these query expressions still eludes me.

EDIT:

I found that you can join implicitly without needing to use the join operator

query {
    for student in db.Student do
    let count = query {
        for course in student.CourseSelection do
        count    
    }

    select (student.Name, count)
}
|> Seq.iter (fun (name, count) -> printfn "%s has %i courses" name count)

F# Query Expression using join & groupBy in same query

3 Answers3