public async Task <IHttpActionResult> Patch(int userId, int elementFieldId, Delta <UserElementField> patch)
        {
            // Owner check: Entity must belong to the current user
            var currentUserId = User.Identity.GetUserId <int>();

            if (currentUserId != userId)
            {
                return(StatusCode(HttpStatusCode.Forbidden));
            }

            // REMARK UserCommandTreeInterceptor already filters "userId" on EntityFramework level, but that might be removed later on / coni2k - 31 Jul. '17
            var userElementField = await _resourcePoolManager.GetUserElementFieldSet(userId, elementFieldId).SingleOrDefaultAsync();

            patch.Patch(userElementField);

            await _resourcePoolManager.SaveChangesAsync();

            return(Ok(userElementField));
        }