docs: def _prepare_order_bys(self):
          """ Build a set of order by columns """
          order_bys = OrderedSet()
          if self._order_bys:
              for ingredient in self._order_bys:
                  if isinstance(ingredient, Dimension):
                      # Reverse the ordering columns so that dimensions
                      # order by their label rather than their id
                      columns = reversed(ingredient.columns)
                  else:
                      columns = ingredient.columns
                  for c in columns:
                      order_by = c.desc() if ingredient.ordering == 'desc' else c
                      if str(order_by) not in [str(o) for o in order_bys]:
                          order_bys.add(order_by)
  
          return list(order_bys)