Properly sort descending=true view results when a key list is provided

Results should now be returned in descending {key, doc_id} order.

The idea is to reverse the key list before sending it to the workers, so they
will emit rows in reverse order. Also, we are using the same reversed list when
building the KeyDict structure on the coordinator. That way the order of the
sent rows and the expected coordinator sorting order will match.

For testing, enhance an existing multi-key Elixir view test to test both
ascending and descending cases and actually check that the rows are in the
correct order each time.

2 files changed, 48 insertions, 10 deletions

diff --git a/src/fabric/src/fabric_view_map.erl b/src/fabric/src/fabric_view_map.erl
index ff6aa8b69..8ab808314 100644
--- a/src/fabric/src/fabric_view_map.erl
+++ b/src/fabric/src/fabric_view_map.erl
@@ -24,8 +24,14 @@ go(DbName, Options, GroupId, View, Args, Callback, Acc, VInfo)
     {ok, DDoc} = fabric:open_doc(DbName, <<"_design/", GroupId/binary>>, []),
     go(DbName, Options, DDoc, View, Args, Callback, Acc, VInfo);
 
-go(Db, Options, DDoc, View, Args, Callback, Acc, VInfo) ->
+go(Db, Options, DDoc, View, Args0, Callback, Acc, VInfo) ->
     DbName = fabric:dbname(Db),
+    Args = case Args0 of
+        #mrargs{keys = Keys, direction = rev} when is_list(Keys) ->
+            Args0#mrargs{keys = lists:reverse(Keys)};
+        #mrargs{} ->
+            Args0
+    end,
     {Shards, RingOpts} = fabric_view:get_shards(Db, Args),
     {CoordArgs, WorkerArgs} = fabric_view:fix_skip_and_limit(Args),
     DocIdAndRev = fabric_util:doc_id_and_rev(DDoc),
@@ -221,10 +227,12 @@ merge_row(Dir, Collation, KeyDict0, Row, Rows0) ->
                             IdA < IdB;
                         {rev, 0} ->
                             IdB < IdA;
-                        {fwd, _} ->
-                            dict:fetch(A, KeyDict1) < dict:fetch(B, KeyDict1);
-                        {rev, _} ->
-                            dict:fetch(B, KeyDict1) < dict:fetch(A, KeyDict1)
+                        {_, _} ->
+                            % We already have a reversed key dict, and sent the
+                            % workers the same reversed keys list. So here we
+                            % just enforce sorting according to the order in
+                            % the key dict
+                            dict:fetch(A, KeyDict1) < dict:fetch(B, KeyDict1)
                     end
                 end,
                 [Row],
diff --git a/test/elixir/test/view_multi_key_design_test.exs b/test/elixir/test/view_multi_key_design_test.exs
index ab57e89eb..c33491620 100644
--- a/test/elixir/test/view_multi_key_design_test.exs
+++ b/test/elixir/test/view_multi_key_design_test.exs
@@ -209,16 +209,38 @@ defmodule ViewMultiKeyDesignTest do
     assert length(rows) == 99
   end
 
-  test "dir works", context do
+  test "dir ascending works", context do
     db_name = context[:db_name]
 
-    resp = view(db_name, "test/multi_emit", [descending: true], [1])
+    expect_rows = mk_rows(0..99, 1, &</2) ++ mk_rows(0..99, 2, &</2)
+
+    resp = view(db_name, "test/multi_emit", [descending: false], [1, 2])
     rows = resp.body["rows"]
-    assert length(rows) == 100
+    assert length(rows) == 200
+    assert expect_rows == rows
 
-    resp = view(db_name, "test/multi_emit", descending: true, keys: :jiffy.encode([1]))
+    keys = :jiffy.encode([1, 2])
+    resp = view(db_name, "test/multi_emit", descending: false, keys: keys)
     rows = resp.body["rows"]
-    assert length(rows) == 100
+    assert length(rows) == 200
+    assert expect_rows == rows
+  end
+
+  test "dir descending works", context do
+    db_name = context[:db_name]
+
+    expect_rows = mk_rows(0..99, 2, &>/2) ++ mk_rows(0..99, 1, &>/2)
+
+    resp = view(db_name, "test/multi_emit", [descending: true], [1, 2])
+    rows = resp.body["rows"]
+    assert length(rows) == 200
+    assert expect_rows == rows
+
+    keys = :jiffy.encode([1, 2])
+    resp = view(db_name, "test/multi_emit", descending: true, keys: keys)
+    rows = resp.body["rows"]
+    assert length(rows) == 200
+    assert expect_rows == rows
   end
 
   test "argument combinations", context do
@@ -313,4 +335,12 @@ defmodule ViewMultiKeyDesignTest do
     rows = resp.body["rows"]
     assert length(rows) == 2
   end
+
+  defp mk_rows(range, key, sort_fun) do
+    row_fun = fn(i) -> %{"id" => "#{i}", "key" => key, "value" => i} end
+    sort_mapper = fn(row) -> {row["key"], row["id"]} end
+    range
+    |> Enum.map(row_fun)
+    |> Enum.sort_by(sort_mapper, sort_fun)
+  end
 end