query.py

def update(self, **kwargs):
  """
  Updates all elements in the current QuerySet, setting all the given
  fields to the appropriate values.
  """
  assert self.query.can_filter(), \
      "Cannot update a query once a slice has been taken."
  meta = self.model._meta
  self._for_write = True
  query = self.query.clone(sql.UpdateQuery)
  query.add_update_values(kwargs)
  extra_data = {}
  if not meta.auto_created:
      responses = signals.pre_update.send(
          sender=self.model, update_fields=kwargs, queryset=self,
          using=self.db)
      for rcvr, response in responses:
          extra_data.update(response)
  with transaction.atomic(using=self.db, savepoint=False):
      rows = query.get_compiler(self.db).execute_sql(CURSOR)
  self._result_cache = None
  if not meta.auto_created:
      signals.post_update.send(
          sender=self.model, update_fields=kwargs,
          extra_data=extra_data, using=self.db)
  return rows