title: Django ORM - No cascading deletion of a many-to-many relationship
body: <p>I'm creating a Django application (using the out of the box ORM) to maintain pizza orders.  I have the following models:</p>
  
  <pre><code>class Employee(models.Model):
      name = models.CharField(max_length=50)
  
  class PizzaPlace(models.Model):
      name = models.CharField(max_length=50)
      employees = models.ManyToManyField(Employee)
  
  class Order(models.Model):
      description = models.CharField(max_length=50)
      pizzaplace = models.ForeignKey(PizzaPlace)
      chef = models.ForeignKey(Employee)
  </code></pre>
  
  <p>I'm using a modelformset_factory to maintain each Pizza Place's employees.  What I want is regardless if an employee is fired or leaves a Pizza Place, the Order they fulfilled remains in tact and is available to the Pizza Place.  What is currently happening is when an Employee record is deleted from a Pizza Place, the delete is cascading.  </p>
  
  <p>What would be the best approach to maintain the association with preserving the Order - particularly maintain that the order was fulfilled, in the past, by the Employee who was deleted even though they are not associated with the Pizza place anymore?</p>