What's the correct way to access linked items using the F# Freebase Type Provider

Question

I have the following piece of code, which aims to list the web sites of stock exchanges listed on Freebase:

#if INTERACTIVE
#r @"C:\Users\kit\Documents\Visual Studio 11\Projects\Demo\packages\FSharpx.TypeProviders.1.7.4\lib\40\FSharpx.TypeProviders.dll"
#r @"C:\Users\kit\Documents\Visual Studio 11\Projects\Demo\packages\FSharpx.TypeProviders.Freebase.1.7.4\lib\40\FSharpx.TypeProviders.Freebase.dll"
#r @"C:\Users\kit\Documents\Visual Studio 11\Projects\Demo\packages\FSharpx.TypeProviders.Freebase.1.7.4\lib\40\FSharpx.TypeProviders.Freebase.DesignTime.dll"
#endif 

let GetExchanges() =
    let dc = FreebaseData.GetDataContext()
    dc.DataContext.SendingRequest.Add(fun e -> printfn "url: %s" e.RequestUri.AbsoluteUri)

    dc.``Products and Services``.Business.``Stock exchanges``
    |> Seq.truncate 10
    |> Seq.iter (fun exchange -> printfn "Exchange: %s" exchange.Name
                                 exchange.``Official website``
                                 |> Seq.iter (fun site -> printfn "%s" site.Name))

Without the final two lines (ie. just listing the Exchange names), the code works fine. With those lines I get a 400 (Bad Request).

The URL generated by that line is this:

https://www.googleapis.com/freebase/v1/mqlread?query=%5B%7B%22/type/object/id%22:null,%20%22/type/object/name%22:null%20,%20%22optional%22:true,%20%22/type/object/type%22:%22/type/uri%22,%20%22!/common/topic/official_website%22:%20%5B%7B%22/type/object/id%22:%22/en/amex%22,%22/type/object/type%22:%22/common/topic%22%20,%20%22limit%22:%20500%7D%5D%7D%5D&cursor

...and if I browse to that I get this from Freebase:

{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "badRequest",
    "message": "Can't reverse /common/topic/official_website as it expects /type/uri, not an object"
   }
  ],
  "code": 400,
  "message": "Can't reverse /common/topic/official_website as it expects /type/uri, not an object"
 }
}

Am I using the correct approach to accessing linked entities? If so, is this a bug in the Type Provider? I get similar errors accessing other linked entities.

Answer 1

This was a bug in the Type Provider that has since been fixed: https://github.com/fsharp/FSharp.Data/issues/68

Answer 2

Like the error says, you can't reverse a property which is primitive value property. You need to turn the query inside out so that it looks like:

[{
  "/type/object/id": "/en/amex",
  "/common/topic/official_website": [{
    "value":    null,
    "optional": true
  }]
}]​

The library must have some knowledge of primitive values, so perhaps it's a simple as adding /type/uri to whatever list it has containing /type/text, /type/rawstring, etc.