Use kind predicates

spec/support/graphql/field_selection.rb

@@ -47,7 +47,7 @@ def serialize_field_selection(hash, level = 0)
     NO_SKIP = ->(_name, _field) { false }
     def self.select_fields(type, skip = NO_SKIP, parent_types = Set.new, max_depth = 3)
-      return new if max_depth <= 0 || !type.respond_to?(:fields) # could be a union!
+      return new if max_depth <= 0 || !type.kind.fields?
       new(type.fields.flat_map do |name, field|
         next [] if skip[name, field]