MyModel.objects.filter(parentcode=ori_code).update(parentcode=new_code)

def updatecategory(request):
  comtype=request.POST.get('comtype','')
  catname=request.POST.get('categoryname','')
  myid=int(request.POST.get('id',''))
  parentid= request.POST.get('parentid','')
  catcode=request.POST.get('categorycode','')
  cat=models.Category.objects.get(autoid=myid)
  ori_parent_code = cat.catcode;
  if cat:
    cat.comtype = comtype
    cat.catname = catname
    cat.parentcode = parentid
    cat.catcode = catcode
    cat.save()
    if cat.parentcode == '-1':          models.Category.objects.filter(parentcode=ori_parent_code).update(parentcode=cat.catcode)