fluent mapping support

[RicoSaupeBosch]

Hi

How do I use the fluent mapping support. I cannot seem to get it to work properly.
I define the mapping below since the object dont have an ID property but that doesnt work when inserting the document to the database.
I I used BSonmapper then the fluent mapping works but it doesnt support the fsharp types.

let createDatabaseUsing (store:Store) (config:AppSettings) =
        let mapper = FSharpBsonMapper()
        mapper.Entity<ServiceRequestAssignment>().Id(fun a -> a.RequestId) |> ignore
        mapper.Entity<SpSite>().Id(fun a -> a.SiteUrl) |> ignore
        
        let db =
            match store with
            | Store.InMemory -> 
                let memoryStream = new MemoryStream()
                new LiteDatabase(memoryStream, mapper)
            | Store.LocalDatabase -> 
                let dbFile = config.Database.File
                new LiteDatabase(dbFile, mapper)
        db

Exception

Could you suggest what I am doing wrong here?