Fix 2 subscriptions bugs

graphql/src/graphql_schema.ml

@@ -1557,11 +1557,16 @@ end
               |> Io.Result.map_error ~f:(fun e -> `Argument_error e) >>=? fun fields ->
               begin match fields with
               | [field] ->
+                if field.name = "__typename" then
+                  (Io.ok (`Response (data_to_json (`Assoc [ (field.name, `String subs.name) ], []))))
+                else
                   (match field_from_subscription_object subs field.name with
                    | Some subscription_field ->
                        (subscribe ctx subscription_field field : ((json, json) result Io.Stream.t, resolve_error) result Io.t :> ((json, json) result Io.Stream.t, [> execute_error]) result Io.t)
                        |> Io.Result.map ~f:(fun stream -> `Stream stream)
-                   | None -> Io.ok (`Response (`Assoc [(alias_or_name field, `Null)])))
+                   | None ->
+                     let err = Printf.sprintf "Field '%s' is not defined on type '%s'" field.name subs.name in
+                     Io.error (`Validation_error err))
               (* see http://facebook.github.io/graphql/June2018/#sec-Response-root-field *)
               | _ -> Io.error (`Validation_error "Subscriptions only allow exactly one selection for the operation.")
               end

graphql/test/introspection_test.ml

@@ -146,4 +146,46 @@ let suite = [
       ]
     ])
   );
+  ("__typename", `Quick, fun () ->
+    let schema = Graphql.Schema.(schema
+      ~query_name:"MyQuery" []
+      ~mutations:[]
+      ~mutation_name:"MyMutation"
+      ~subscriptions:[]
+      ~subscription_name:"MySubscription"
+    ) in
+    test_query schema
+      {|
+        query {
+          __typename
+        }
+      |}
+      (`Assoc [
+        "data", `Assoc [
+          "__typename", `String "MyQuery"
+        ]
+      ]);
+    test_query schema
+      {|
+        mutation {
+          __typename
+        }
+      |}
+      (`Assoc [
+        "data", `Assoc [
+          "__typename", `String "MyMutation"
+        ]
+      ]);
+    test_query schema
+      {|
+        subscription {
+          __typename
+        }
+      |}
+      (`Assoc [
+        "data", `Assoc [
+          "__typename", `String "MySubscription"
+        ]
+      ])
+  )
 ]

graphql/test/schema_test.ml

@@ -336,5 +336,15 @@ let suite = [
         ]
       ]
     ])
+  );
+  ("subscription field that doesn't exist", `Quick, fun () ->
+    let query = "subscription { dont_exist { id name } }" in
+    test_query query (`Assoc [
+      "errors", `List [
+        `Assoc [
+          "message", `String "Field 'dont_exist' is not defined on type 'subscription'"
+        ]
+      ]
+    ])
   )
 ]