Undefined column selection on polymorphic table queries

[Luke265]

create table relational_items (
  id serial primary key,
  type item_type not null default 'TOPIC'::item_type,
  deleted_at timestamp with time zone
);

create table relational_topics (
  id int primary key references relational_items,
  title text not null
);
comment on table relational_items is $$
  @interface mode:relational type:type
  @type TOPIC references:relational_topics
  $$;

query MyQuery {
  allRelationalTopics {
    nodes {
      title
    }
  }
}

query returns error error: column __relational_topics__.deleted_at does not exist on postgraphile V5

full reproduction: benjie/ouch#45