helper for add_missing_children. sets k:None for the field for pkeys not already in request.

def merge_null_missing(request,name,field,pkeys):
  "helper for add_missing_children. sets k:None for the field for pkeys not already in request."
  request.update({rk:None for rk in [FieldKey(name,pkey,field) for pkey in pkeys] if rk not in request})