queries over data provided by ODataService type providers

[jamescheney]

Queries

For example, the following simple query over the Northwind database provided by ODataService fails. (Additional examples are in the new file NorthwindTests.)

open Microsoft.FSharp.Data.TypeProviders;

type Northwind = ODataService<"http://services.odata.org/Northwind/Northwind.svc">
let db = Northwind.GetDataContext()

// Some tests to compare 

let dbQuery =  FSharpComposableQuery.TopLevelValues.query

dbQuery { for x in db.Customers do yield x }

The error message is:

System.Exception: Unexpected table reference PropertyGet (Some (PropertyGet (None, db, [])), Customers, []) System.Data.Services.Client.DataServiceQuery`1[UnitTestProject1+Northwind+ServiceTypes+Customer]
Result StackTrace:  
at FSharpComposableQuery.QueryImpl.from@464-2.Invoke(String message)
   at Microsoft.FSharp.Core.PrintfImpl.go@523-3[b,c,d](String fmt, Int32 len, FSharpFunc`2 outputChar, FSharpFunc`2 outa, b os, FSharpFunc`2 finalize, FSharpList`1 args, Int32 i)
   at Microsoft.FSharp.Core.PrintfImpl.run@521[b,c,d](FSharpFunc`2 initialize, String fmt, Int32 len, FSharpList`1 args)
   at Microsoft.FSharp.Core.PrintfImpl.capture@540[b,c,d](FSharpFunc`2 initialize, String fmt, Int32 len, FSharpList`1 args, Type ty, Int32 i)
   at .$Reflect.Invoke@720-4.Invoke(T1 inp)
   at FSharpComposableQuery.QueryImpl.handleSpecificCall@488(QueryBuilder this, FSharpOption`1 obj, MethodInfo func, FSharpList`1 args, Type expr_ty)
   at FSharpComposableQuery.QueryImpl.handleSpecificCall@494-2.Invoke(Type ty, Type _arg4)
   at FSharpComposableQuery.QueryImpl.QueryBuilder.Norm[T](FSharpExpr`1 expr)
   at FSharpComposableQuery.QueryImpl.QueryBuilder.Run[T](FSharpExpr`1 q)

[jamescheney]

This exception is being raised by the following code in Query.fs:

| Patterns.PropertyGet(Some(Variable(None, _db)), _tbl, []) as e -> 
                    match e.Type with
                    | QuerySourceTy(ty, _) -> Table(e, ty)  // assume it's a db table ref
                    | TableTy ty -> Table(e, ty)            // assume it's a db table ref
                    | ty' -> failwithf "Unexpected table reference %A %A" e ty'

It looks like we need to recognize [System.Data.Services.Client.DataServiceQuery`1[UnitTestProject1+Northwind+ServiceTypes+Customer]] as an allowed table type. Is there a good way of recognizing all of the possible table types?

[jamescheney]

Now fixed.

[ixtreon]

Although the fix addresses the specific issue, I believe it is highly specific to the System.Data.Services.Client.DataServiceQuery<T> type.

First off, the exception occurs as the only variable references valid in a query are those querying a database table, and the DataServiceQuery<T> type is not recognized as such.

Before this issue was posted the code had checked if a variable was of type Microsoft.Fsharp.Linq.QuerySource<T,Q> or System.Data.Linq.Table<T> to determine if it is a table, and commit a0b1789 added an explicit check to test if a variable is of type DataServiceQuery<T> (along with an extra reference for the library).

I observed that both DataServiceQuery<T> and Table<T> implement the System.Linq.IQueryable<T> interface which is also the base unwrapped type all query methods work with. On the other hand, the Microsoft.Fsharp.Linq.QuerySource<T,Q> type is the wrapped type they use.

I thus believe checking for those two types, including for subtypes of the former, would be both correct and sufficient.