Added OfTypes case to ObjectListFilter

Author: xperiandri

src/FSharp.Data.GraphQL.Server.Middleware/ObjectListFilter.fs

@@ -1,5 +1,7 @@
 namespace FSharp.Data.GraphQL.Server.Middleware
 
+open System
+
 /// A filter definition for a field value.
 type FieldFilter<'Val> =
     { FieldName : string
@@ -16,10 +18,10 @@ type ObjectListFilter =
     | StartsWith of FieldFilter<string>
     | EndsWith of FieldFilter<string>
     | Contains of FieldFilter<string>
+    | OfTypes of FieldFilter<Type list>
     | FilterField of FieldFilter<ObjectListFilter>
     | NoFilter
 
-
 /// Contains tooling for working with ObjectListFilter.
 module ObjectListFilter =
     /// Contains operators for building and comparing ObjectListFilter values.

src/FSharp.Data.GraphQL.Server.Middleware/TypeSystemExtensions.fs

@@ -31,7 +31,17 @@ module TypeSystemExtensions =
         /// </summary>
         member this.Filter =
             match this.Args.TryGetValue "filter" with
-            | true, (:? ObjectListFilter as f) -> ValueSome f
+            | true, (:? ObjectListFilter as f) ->
+                match this.ExecutionInfo.Kind with
+                | ResolveAbstraction typeFields ->
+                    let getType name =
+                        match this.Context.Schema.TypeMap.TryFind name with
+                        | ValueSome tdef -> tdef.Type
+                        | ValueNone -> raise (MalformedGQLQueryException ($"Type '{name}' not found in schema."))
+                    match typeFields.Keys |> Seq.map getType |> Seq.toList with
+                    | [] -> ValueNone
+                    | filters -> f &&& (OfTypes { FieldName = "__typename"; Value = filters }) |> ValueSome
+                | _ -> ValueSome f
             | false, _ -> ValueNone
             | true, _ -> raise (InvalidOperationException "Invalid filter argument type.")