How transform specific geometry column using Django GIS GeoQuerySet

I am trying to transform geometry column type in django, here is my model

class Network(models.Model):      name = models.CharField(max_length=50, blank=True)      alias = models.CharField(max_length=100, blank=True)      geometry = models.GeometryField(srid=3857, null=True, blank=True)      bbox = models.GeometryField(srid=3857, null=True, blank=True)      objects = models.GeoManager()        class Meta:          db_table = u'tbl_network'        def __unicode__(self):          return '%s' % self.name

Problem is that when i tried to transform bbox it does not transform it

  Network(srid=3857).values('geometry','bbox')      SELECT ST_Transform("tbl_network"."geometry", 3857), "tbl_network"."bbox" FROM "tbl_network"

Above query automaticly tranform geomtery field and igonre bbox field

So i tried it different different way

 Network(srid=3857).values('bbox')     SELECT "tbl_network"."bbox" FROM "tbl_network"

Now this query totally ignored transform function.

So my questions is,

How can i transform specific column of model?

thank you

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