code: def extent(self, srid=None):
          
          expr = self.geo_field.name
          if srid:
              expr = geofn.Transform(expr, srid)
          expr = models.Extent(expr)
          clone = self.all()
          name, val = clone.aggregate(expr).popitem()
          return val
docs: Returns the GeoQuerySet extent as a 4-tuple.
  
          Keyword args:
          srid -- EPSG id for for transforming the output geometry.