Unions in Graphile?

schema.graphql

type User {
  id: ID!
  name: String!
}

type Group {
  id: ID!
  name: String!
  school: School!
}

type School {
  id: ID!
  name: String!
  organization: Organization!
}

type Organization {
  id: ID!
  name: String!
}

union Owner = User | Group | School | Organization

type BlogPost {
  id: ID!
  title: String
  body: String
  author: User!
  owner: Owner
}

schema.sql

/**
 * This is simplified and I've omitted smart comments etc that'd clean things up. Use your imagination ;)
 */
 
create table user (
  id serial primary key,
  name text not null
);

create table group (
  id serial primary key,
  name text not null,
  school integer not null references school(id)
);

create table school (
  id serial primary key,
  name text not null,
  organization integer not null references organization(id)
);

create table organization (
  id serial primary key,
  name text not null
);

create table blog_post (
  id serial primary key,
  title text,
  body text,
  author integer not null references user(id),
  owning_user integer references user(id),
  owning_group integer references group(id),
  owning_school integer references school(id),
  owning_organization integer references organization(id)
);