+ # If the client-specified orders don't amount to a full ordering
+ # (e.g., [] or ['owner_uuid desc']), fall back on the default
+ # orders to ensure repeating the same request (possibly with
+ # different limit/offset) will return records in the same order.
+ #
+ # Clean up the resulting list of orders such that no column
+ # uselessly appears twice (Postgres might not optimize this out
+ # for us) and no columns uselessly appear after a unique column
+ # (Postgres does not optimize this out for us; as of 9.2, "order
+ # by id, modified_at desc, uuid" is slow but "order by id" is
+ # fast).
+ orders_given_and_default = @orders + model_class.default_orders
+ order_cols_used = {}
+ @orders = []
+ orders_given_and_default.each do |order|
+ otablecol = order.split(' ')[0]
+
+ next if order_cols_used[otablecol]
+ order_cols_used[otablecol] = true
+
+ @orders << order
+
+ otable, ocol = otablecol.split('.')
+ if otable == table_name and model_class.unique_columns.include?(ocol)
+ # we already have a full ordering; subsequent entries would be
+ # superfluous
+ break
+ end