def _SortableApprovalApproverValues(art, fd_list, users_by_id):
  """Return a list of approval approvers relevant to one UI table column."""
  sortable_value_list = []
  for fd in fd_list:
    for av in art.approval_values:
      if av.approval_id == fd.field_id:
        sortable_value_list.extend(
          [users_by_id.get(approver_id).email
           for approver_id in av.approver_ids
           if users_by_id.get(approver_id)])

  return sortable_value_list