title: Postgres FK referencing composite PK
body: <p>Consider </p>
  
  <pre><code>CREATE TABLE foo (
      id SERIAL,
      foo_created_on ABSTIME,
      foo_deactivated_on ABSTIME,
      PRIMARY KEY (id, foo_created_on)
  );
  
  CREATE TABLE bar (
      id SERIAL,
      bar_created_on ABSTIME,
      bar_deactivated_on ABSTIME,
      foo_id REFERENCES ( .. what goes here? ..),
      PRIMARY KEY (id, bar_created_on)
  );
  </code></pre>
  
  <p>How do I create an FK in "bar" that references the PK in "foo"?</p>