title: how can i add foreign key to existing class in django
body: <p>I have two simple models of country and town:</p>
  <pre><code>from django.db import models
  
  class Country(models.Model):
      name = models.CharField(max_length=50)
      
  class Town(models.Model):
      belongs_to = models.ForeignKey(country, on_delete=models.CASCADE)
      name = models.CharField(max_length=50)
  </code></pre>
  <p>Now I would like to play around a bit... and I would like to add continent. I guess correct way from the start would be to do this:</p>
  <pre><code>from django.db import models
  
  class Continent(models.Model):
      name = models.CharField(max_length=50)
  
  class Country(models.Model):
      belongs_to = models.ForeignKey(continent, on_delete=models.CASCADE)
      name = models.CharField(max_length=50)
      
  class Town(models.Model):
      belongs_to = models.ForeignKey(country, on_delete=models.CASCADE)
      name = models.CharField(max_length=50)
  </code></pre>
  <p>If I do that and save the file, add continents in admin.py to admin.site.register(continent), start with makemigrations... I get this:</p>
  <p>You are trying to add a non-nullable field 'belongs' to country without a default; we can't do that (the database needs som
  ething to populate existing rows).
  Please select a fix:</p>
  <p>1.) Provide a one-off default now (will be set on all existing rows
  with a null value for this column)  2.) Quit, and let me add a
  default in models.py</p>
  <p>I get stuck here, because I do not know what to do from here?</p>