title: using property for django fields causes infinite loop
body: <p>I'm trying to make fields in my model write once and read only there after. The solution I came up with is using the property decorators. Please tell me if there is a better solution, I'm new to django. I get into an infinite loop when I try to instantiate the model in the django shell.</p>
  
  <pre><code>class MapPointable(models.Model):
  loc_latitude = models.FloatField(null = True)
  loc_longtitude = models.FloatField(null = True)
  
  @property
  def loc_latitude(self):
      return self.loc_latitude
  
  @loc_latitude.setter
  def loc_latitude(self, value):
      if self.loc_latitude == None:
          self.loc_latitude = value
      else:
          raise ValueError("Read-only field, the value cannot be set")
  
  @property
  def loc_longtitude(self):
      return self.loc_longtitude
  
  @loc_longtitude.setter
  def loc_longtitude(self, value):
      if self.loc_longtitude == None:
          self.loc_longtitude = value
      else:
          raise ValueError("Read-only field, the value cannot be set")
  </code></pre>