returns {ModelClass:list_of_pkey_tuples}. see syncschema.RefKey. Don't use this yet.

def refkeys(self,fields):
    "returns {ModelClass:list_of_pkey_tuples}. see syncschema.RefKey. Don't use this yet."
    # todo doc: better explanation of what refkeys are and how fields plays in
    dd=collections.defaultdict(list)
    if any(f not in self.REFKEYS for f in fields): raise ValueError(fields,'not all in',self.REFKEYS.keys())
    for f in fields:
      rk=self.REFKEYS[f]
      for model in rk.refmodels: dd[model].extend(rk.pkeys(self,f))
    return dd