Partitions with custom reduce are brokenbroken-partitioned-custom-reduce

```
make elixir tests=test/elixir/test/partition_view_test.exs:314

  * test query with custom reduce works (708.2ms)

  1) test query with custom reduce works (ViewPartitionTest)
     test/elixir/test/partition_view_test.exs:314
     Assertion with == failed
     code:  assert resp.status_code == 200
     left:  500
     right: 200
     stacktrace:
       test/elixir/test/partition_view_test.exs:319: (test)

```

Logs show:

```
[error] 2022-03-11T20:05:58.969707Z node1@127.0.0.1 <0.985.0> 9bc85227db req_err(2412564908) {{invalid_ejson,{p,<<"bar">>,[<<"field">>,<<"one">>]}},
 [{jiffy,encode,2,
         [{file,"/Users/nvatama/asf-3-main/src/jiffy/src/jiffy.erl"},
          {line,99}]},
  {couch_os_process,writejson,2,[{file,"src/couch_os_process.erl"},{line,97}]},
  {couch_os_process,handle_call,3,
```

1 files changed, 40 insertions, 0 deletions

diff --git a/test/elixir/test/partition_view_test.exs b/test/elixir/test/partition_view_test.exs
index 0a55c2443..7edd79b44 100644
--- a/test/elixir/test/partition_view_test.exs
+++ b/test/elixir/test/partition_view_test.exs
@@ -29,6 +29,16 @@ defmodule ViewPartitionTest do
       }
     """
 
+    red_custom = """
+        function(keys, values, rereduce) {
+            if (rereduce) {
+                return sum(values);
+            } else {
+                return values.length;
+            }
+        }
+     """
+
     query = %{:w => 3}
 
     body = %{
@@ -56,6 +66,10 @@ defmodule ViewPartitionTest do
           views: %{some: %{map: map_fun2, reduce: "_count"}}
         },
         %{
+          _id: "_design/reduce_custom",
+          views: %{some: %{map: map_fun2, reduce: red_custom}}
+        },
+        %{
           _id: "_design/include_ddocs",
           views: %{some: %{map: map_fun1}},
           options: %{include_design: true}
@@ -297,6 +311,32 @@ defmodule ViewPartitionTest do
            ]
   end
 
+  test "query with custom reduce works", context do
+    db_name = context[:db_name]
+
+    url = "/#{db_name}/_partition/foo/_design/reduce_custom/_view/some"
+    resp = Couch.get(url, query: %{reduce: true, group_level: 1})
+    assert resp.status_code == 200
+    results = get_reduce_result(resp)
+    assert results == [%{"key" => ["field"], "value" => 50}]
+
+    resp = Couch.get(url, query: %{reduce: true, group_level: 2})
+    results = get_reduce_result(resp)
+
+    assert results == [
+             %{"key" => ["field", "one"], "value" => 16},
+             %{"key" => ["field", "two"], "value" => 34}
+           ]
+
+    resp = Couch.get(url, query: %{reduce: true, group: true})
+    results = get_reduce_result(resp)
+
+    assert results == [
+             %{"key" => ["field", "one"], "value" => 16},
+             %{"key" => ["field", "two"], "value" => 34}
+           ]
+  end
+
   test "partition query can set query limits", context do
     set_config({"query_server_config", "partition_query_limit", "2000"})