title: You are trying to add a non-nullable field 'id' to contact_info without a default
body: <p>I am using the command <code>python manage.py makemigrations</code></p>
  <p>However, I get this error:</p>
  <blockquote>
  <pre class="lang-none prettyprint-override"><code>You are trying to add a non-nullable field 'id' to contact_info
  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>
  </blockquote>
  <p>Here is models.py:</p>
  <pre><code>class Document(models.Model):
      docfile = models.FileField(upload_to='documents/')
  
  
  class contact_number(models.Model):
      contact_numbers = models.CharField(max_length=13)
  
  class contact_address(models.Model):
      address = models.CharField(max_length=100)
      city = models.CharField(max_length=50)
      state = models.CharField(max_length=50)
      zip = models.CharField(max_length=5)
  
  class contact_info(models.Model):
      first_name = models.CharField(max_length=50)
      last_name = models.CharField(max_length=50)
      email = models.EmailField()
      contact_numbers = models.ManyToManyField(contact_number)
      addresses = models.ManyToManyField(contact_address)
  </code></pre>