docs: def get_id_constraints(pkname, pkey):
    """Returns primary key consraints.
  
    :pkname: if a string, returns a dict with pkname=pkey. pkname and pkey must be enumerables of
    matching length.
    """
    if isinstance(pkname, str):
      return {pkname: pkey}
    else:
      return dict(zip(pkname, pkey))