title: Django model booleanfield only 1 True for table
body: <p>I have the model</p>
  
  <pre><code>class Owner(models.Model):
      name = models.CharField(max_length=10)
  
  class PhoneNumber(models.Model):
      isActive = model.BooleanField(default=False)
      owner = model.ForeignKey('Owner')
  </code></pre>
  
  <p>I want to enforce that only one PhoneNumber is active so if the user, when creating or editing a 'not active' PhoneNumber, accidentally sets it to 'active' and there is another 'active' PhoneNumber already the form should not submit and a clear error will get displayed to change the 'active' field to False and error text "Please deactivate the old active PhoneNumber before assigning a new active PhoneNumber"</p>
  
  <p>How do I do this? In which validation method in which class do I check this easily ?</p>
  
  <p>Thank you</p>