title: Filter related models by substring
body: <p>Let's say I have the following models</p>
  
  <pre><code>class PizzaOrder(models.Model):
      order = models.CharField(max_length=200)
  
  class Topping(models.Model)
      topping = models.CharField(max_length=50)
  </code></pre>
  
  <p>and a <code>PizzaOrder</code> like this</p>
  
  <pre><code>order = PizzaOrder.objects.create(
      order='large pizza with onions, mushrooms and tuna'
  )
  </code></pre>
  
  <p>Is it possible to perform a single query through django's ORM to get all the toppings used in this <code>PizzaOrder</code>?</p>
  
  <p>This works but is expensive:</p>
  
  <pre><code>for topping in Topping.objects.all():
     if topping.topping in order.order:
        print("This order has a {} topping".format(topping.topping))
  </code></pre>