title: Distinct in many-to-many relation
body: <pre><code>class Order(models.Model):
      ...
  
  class OrderItem(models.Model)
      order = models.ForeignKey(Order)
      product = models.ForeignKey(Product)
      quantity = models.PositiveIntegerField()
  </code></pre>
  
  <p>What I need to do is to get the Order(s) which has only one order item. How can I write it using the QuerySet objects (without writing SQL)?</p>