title: How can I set the field unique in django?
body: <p>I have a model class:</p>
  
  <pre><code>class PysicalServer(models.Model):
  
      serial_number = models.CharField(max_length=64)  # I want to add the unique
      name = models.CharField(max_length=16)
  </code></pre>
  
  <p>I know use the primary_key can set unique, but the serial_number is not my id field, I can not use the primary_key, is there other property I can set field unique?</p>