title: SilverStripe 4 - has_many relation on DataExtension
body: <p>On SS 4.0.3 I'm just trying to make a relation between a <code>SiteConfig</code> extension and a <code>DataObject</code>. The <code>DataExtension</code> has a <code>has_many</code> relation pointing to a correspondent <code>has_one</code> in the <code>DataObject</code>. 
  On <code>/dev/build?flush=all</code> the CMS throw me the following exception, and I don't understand why. </p>
  
  <p>To be precise; <code>DataObject</code> as <code>Cassa</code> and <code>DataExtension</code> as <code>ConfigurazioneExtension</code>:</p>
  
  <p>This is the mesage:</p>
  
  <blockquote>
    <p>[Emergency] Uncaught Exception: No has_one found on class 'Cassa', the has_many relation from 'SilverStripe\SiteConfig\SiteConfig' to 'Cassa' requires a has_one on 'Cassa'</p>
  </blockquote>
  
  <p>Here's the code:</p>
  
  <p><strong>Cassa</strong></p>
  
  <pre><code>/**
   * Classe Cassa
   */
  class Cassa extends MetodoPagamento
  {
      // Dichiarazione Proprietà
      private static $db = [
          'Iban' =&gt; 'Varchar',
          'IstitutoCredito' =&gt; 'Text'
      ];
      private static $has_one = [
          'ConfigurazioneExtension' =&gt; 'ConfigurazioneExtension'
      ]; // etc...
  </code></pre>
  
  <p><strong>ConfigurazioneExtension</strong></p>
  
  <pre><code>/**
   * Classe Configurazione - Estensione
   */
  class ConfigurazioneExtension extends DataExtension
  {
      // Dichiarazione Proprietà
      private static $has_many = [
          'Casse' =&gt; 'Cassa'
      ]; // etc...
  </code></pre>
  
  <p>Are some types of relations prohibited between DataObjects and DataExtensions?</p>
  
  <p>Thanks in advance for your help.</p>