title: Django ORM Inheritance: You are trying to add a non-nullable field 'dish_ptr' to [...] without a default
body: <p>I am working on a restaurant app (and new to Django/Python). I want to have a parent class Dish that will contain some counter or ID that increments for every instance of a child class of Dish. The instances are dishes like Pizza, Pasta, etc with different characteristics and relations. I'm trying to make <code>Dish</code> concrete, because in that case I reckon I just access <code>Dish</code>'s PK and that will give each menu-item a Dish-ID. 
  However, I don't know how to correctly fix the errors I come across: <code>You are trying to add a non-nullable field 'pasta_ptr'</code>. </p>
  
  <p>Here are the relevant code snippets:</p>
  
  <pre><code>class Dish(models.Model):
    pass # should automatically generate PK, right?
  
  class Pasta(Dish):
    name = models.CharField(max_length=64, primary_key=True)
    price = models.DecimalField(max_digits=6, decimal_places=2)
  
    def __str__(self):
      return f"{self.name}, price: ${self.price}"
  
  class Pizza(Dish):
    sizestyle = models.CharField(max_length=4, choices=SIZESTYLE_CHOICES, default=SMALL_REGULAR)
    topping_count = models.IntegerField(default=0, validators=[MaxValueValidator(5), MinValueValidator(0)])
    price = models.DecimalField(max_digits=6, decimal_places=2)
  
    def __str__(self):
      return f"Price for {self.sizestyle} pizza with {self.topping_count} toppings: ${self.price}"
  
  class Sub(Dish):
    name = models.CharField(max_length=64, primary_key=True)
    price_category = models.ForeignKey(SubPrice, on_delete=models.DO_NOTHING, related_name="sub_price_category")
  
    def __str__(self):
      return f"{self.name}, Price Category: {self.price_category}"
  
  class Platter(Dish):
    name = models.CharField(max_length=64, primary_key=True)
    price_large = models.DecimalField(max_digits=6, decimal_places=2, default=0)
    price_small = models.DecimalField(max_digits=6, decimal_places=2, default=0)
  
    def __str__(self):
      return f"{self.name} price: Large ${self.price_large}, Small ${self.price_small}"
  </code></pre>