title: construct condition in save model in django admin
body: <p>I have Contact model and I want to construct it to have only 1 record with field is_active=True.
  but I don't want to raise error, I want it to notify me and don't save the record if there is an contact record with is_active=True before in admin site.
  "Contact can't have more than 1 active contact at same time."</p>
  
  <pre><code>class Contact(models.Model):
      name = models.CharField(max_length=30)
      is_active = models.BooleanField(blank=True)
  
      def save(self, *args, **kwargs):
          if self.is_active:
              if Contact.objects.filter(Q(is_active=True), ~Q(id=self.id)):
              // raise ValidationError("Contact can't have more than 1 active contact at same time.")
          return super(Contact, self).save(*args, **kwargs)
  </code></pre>