title: Qweb how to select records with non-null field
body: <p>I have a QWeb report where object <code>o</code> is <code>stock.move(97,99,98,325,326,327)</code></p>
  
  <p>so</p>
  
  <pre><code>&lt;span t-field="o"/&gt;
  </code></pre>
  
  <p>displays</p>
  
  <pre><code>stock.move(97,99,98,325,326,327)
  </code></pre>
  
  <p>In database, from all those <code>stock.move</code> elements, only three of them have populated the field <code>procurement_id</code>:</p>
  
  <pre><code># select id,procurement_id from stock_move where id in (97,98,99,325,326,327);
   id  | procurement_id
  -----+----------------
    97 |
    98 |
    99 |
   325 |             16
   326 |             17
   327 |             18
  (6 rows)
  </code></pre>
  
  <p>If I do:</p>
  
  <pre><code>&lt;span t-field="o.procurement_id"/&gt;
  </code></pre>
  
  <p>The QWeb report is not generated, probably because we are trying to retrieve a null instance of <code>procurement.order</code> (in this case, three null instances).</p>
  
  <p>How can I retrieve just the three existing <code>procurement.order</code> instances , i.e. the ones that have populated <code>procurement_id</code>?</p>