title: HQL left join where left is null
body: <p>how to write this query on HQL:</p>
  
  <pre><code>select pp.* 
  from Part pp
   left join Product p on pp.ProductID = p.ID
  where p.ID is null
  </code></pre>
  
  <p>i need parts without products. Part has property Product (many-to-one)</p>
  
  <p>I tried</p>
  
  <pre><code>from Part p 
  where p.Product is null
  </code></pre>
  
  <p>but it generate invalid query.</p>
  
  <p>Thanks</p>