title: How to store unregistered users in database?
body: <p>I have a table that contains users of my system. The <code>order</code> table has columns:</p>
  <pre><code>id | user_id | price
  </code></pre>
  <p>Where <code>user_id</code> is foreign key.</p>
  <p>Problem is that if user not registered there is no <code>user_id</code>. It means order cannot be placed because violates the integrity.</p>