title: django.db.utils.IntegrityError: NOT NULL constraint failed: app.area_id
body: <p>When I run <code>./manage.py migrate</code>,error happens <code>django.db.utils.IntegrityError: NOT NULL constraint failed: app.area_id</code> .
  <code>models.py</code> is </p>
  
  <pre><code>class Area(models.Model):
      name = models.CharField(max_length=20, verbose_name='area', null=True)
  
  class User(models.Model):
      name = models.CharField(max_length=200,null=True)
      age = models.CharField(max_length=200,null=True)
      area = models.ForeignKey('Area', default="")
  
  class Prefecture(models.Model):
      name = models.CharField(max_length=20, verbose_name='city')
      area = models.ForeignKey('Area')
  
  class City(models.Model):
      name = models.CharField(max_length=20, verbose_name='region')
              prefecture = models.ForeignKey('Prefecture')
  
  class Price(models.Model):
      name = models.CharField(max_length=20, verbose_name='price')
              PRICE_RANGE = (
                  ('a', 'under500'),
                  ('b', '500-1000'),
                  ('c', 'upper1000'),
              )
      price_range = models.CharField(max_length=1, choices=PRICE_RANGE)
      city = models.ForeignKey('City')
  </code></pre>
  
  <p>When I wrote <code>area = models.ForeignKey('Area')</code>,I got an error 
  You are trying to add a non-nullable field 'area' to transaction without a default; we can't do that (the database needs something to populate existing rows). Please select a fix: 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 <code>models.py</code>.
  What is wrong? How can I fix this?</p>
  
  <p>Now models.py is</p>
  
  <pre><code>class Area(models.Model):
      name = models.CharField(max_length=20, verbose_name='area', null=True)
  
  class User(models.Model):
      name = models.CharField(max_length=200,null=True)
      age = models.CharField(max_length=200,null=True)
      area = models.ForeignKey('Area', default="")
  
  class Prefecture(models.Model):
      name = models.CharField(max_length=20, verbose_name='city')
      area = models.ForeignKey('Area', null=True, blank=True)
  
  class City(models.Model):
      name = models.CharField(max_length=20, verbose_name='region')
              prefecture = models.ForeignKey('Prefecture', null=True, blank=True)
  
  class Price(models.Model):
      name = models.CharField(max_length=20, verbose_name='price')
              PRICE_RANGE = (
                  ('a', 'under500'),
                  ('b', '500-1000'),
                  ('c', 'upper1000'),
              )
      price_range = models.CharField(max_length=1, choices=PRICE_RANGE)
      city = models.ForeignKey('City', null=True, blank=True)
  </code></pre>
  
  <p>I also got same error.I already tried cache.clear() .</p>