title: overwriting Django Model
body: <p>i need to update the columnvalue of a particular field everytime i create a new entry to the table with some hash value.
  i created the model as follows:</p>
  
  <pre><code>class ICOExchange(models.Model):
  
      id = models.CharField(default=create_UUID('user '),
                            max_length=100, primary_key=True)
  
  
      def set_id(self):
          self.id = create_UUID('user ')
  </code></pre>
  
  <p>where <code>create_UUID(arg)</code> is a function that generates a hash value according to some inputs and timestamp.
  But while adding datasets through the admin panel it doesnt refresh the id and id remains the same.</p>