title: How to add a temporary field for calculation in django model?
body: <p>I am developing a GIS based django app in which i am allocating the nearest exam center to a student based on his current location.
  I have my centre model like this</p>
  
  <pre><code> class Center(models.Model):
      name = models.CharField(max_length=500)
      city = models.CharField(max_length=100)
      state = models.CharField(max_length=100)
      latitude = models.FloatField()
      longitude = models.FloatField()
      capicity = models.IntegerField()
      examid = models.ForeignKey(Exam)
  
      class Meta:
          ordering = ['distance']
  </code></pre>
  
  <p>I am using a function which accept the user's current location and city and then calculate the distance between the current location and all available centres in the given city. And then sort them by distance. 
  But i don't want to change my centre model and use the distance attribute as a temporary attribute for the existing centre model.</p>