title: Django error "add a non-nullable field"
body: <p>i am getting error django model like this when i try to makemigrations:</p>
  
  <pre><code>    You are trying to add a non-nullable field 'person' to owner 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)
   2) Quit, and let me add a default in models.py
  </code></pre>
  
  <p>i use django 1.8 and this is my model:</p>
  
  <pre><code>class Person(models.Model):
      user            = models.OneToOneField(User)
      alphanumeric    = RegexValidator(r'^[0-9a-zA-Z]*$', message='hanya yang mengandung karakter alphanumeric')
      email           = models.EmailField(verbose_name='email address', unique=True, max_length=244)
      username        = models.CharField(unique=True, max_length=20, validators=[alphanumeric])
      first_name      = models.CharField(max_length=30, null=True, blank=True)
      last_name       = models.CharField(max_length=30, null=True, blank=True)
      date_of_birth   = models.DateTimeField()
      date_joined     = models.DateTimeField(auto_now_add=True)
  
  
      USERNAME_FIELD      = 'username'
      REQUIRED_FIELDS     = ['email']
  
      def get_full_name(self):
          fullname = self.first_name+" "+self.last_name
          return self.fullname
  
      def get_short_name(self):
          return self.username
  
      def list_operator(self):
          return self.operators.all()
  
      def __str__(self):
          return self.email
  
  class Operator(models.Model):
      person          = models.ForeignKey(Person, related_name="operators", null=True)
      alphanumeric    = RegexValidator(r'^[0-9a-zA-Z]*$', message='hanya yang mengandung karakter alphanumeric')
      email           = models.EmailField(verbose_name='email address', unique=True, max_length=244)
      username        = models.CharField(unique=True, max_length=20, validators=[alphanumeric])
      first_name      = models.CharField(max_length=30, null=True, blank=True)
      last_name       = models.CharField(max_length=30, null=True, blank=True)
      date_of_birth   = models.DateTimeField(auto_now_add=True)
  
      def __str__(self):
          return self.username;
  </code></pre>
  
  <p>i want to know where my code is wrong. 
  can you help me solved this problem?</p>