title: Silverstripe creates only Indices but no Foreign Keys
body: <p>I am using the PostgresSQL module for SilverStripe.</p>
  
  <p>My issue is that I can define relations with for example: </p>
  
  <pre><code>public static $has_one = array( 
  'PageRedirect' =&gt; 'SiteTree', 
  );
  </code></pre>
  
  <p>What this code will do is that it will create the database with a Indice with the naming pattern: ix_tablename_pageredirectId 
  However no foreign key is created.</p>
  
  <p>Further SilverStripe seems to write "0" instead of "null" in columns that are empty relations. So actually you can't even create those foreign key constraints as SilverStripe will constantly violate them.</p>
  
  <p>How can one configure SilverStripe for proper Foreign Key Constraints in the database?</p>
  
  <p>This is SilverStripe v2.4.x</p>