Circular FK constraints in postgresql

gistfile1.txt

-- Three tables: books, authors, authorship 
-- Every book must have an author, enforce this constraint at schema level

drop table if exists book cascade;
drop table if exists author cascade;
drop table if exists authorship cascade;

create table book(id serial not null primary key);
create table author(id serial not null primary key);
create table authorship(
  book_id integer not null unique references book(id), 
  author_id integer not null references author(id), 
  primary key (book_id, author_id)
);
alter table book 
  add constraint book_must_have_author 
  foreign key (id) references authorship(book_id) initially deferred;