Fix filter transformations (plausible#4727)

Previously the code would use the incorrect operator and lead to nesting

lib/plausible/stats/filters/filters.ex

@@ -99,8 +99,8 @@ defmodule Plausible.Stats.Filters do
 
     filters
     |> traverse()
-    |> Enum.filter(fn {_filter, _root, depth} -> depth >= min_depth end)
-    |> Enum.map(fn {[_operator, dimension | _rest], _root, _depth} -> dimension end)
+    |> Enum.filter(fn {_filter, depth} -> depth >= min_depth end)
+    |> Enum.map(fn {[_operator, dimension | _rest], _depth} -> dimension end)
   end
 
   def filtering_on_dimension?(query, dimension) do
@@ -145,7 +145,8 @@ defmodule Plausible.Stats.Filters do
     case {transformer.(filter), filter} do
       # Transformer did not return that value - transform that subtree
       {nil, [operation, child_filter]} when operation in [:not, :ignore_in_totals_query] ->
-        [[:not, transform_tree(child_filter, transformer)]]
+        [transformed_child] = transform_tree(child_filter, transformer)
+        [[operation, transformed_child]]
 
       {nil, [operation, filters]} when operation in [:and, :or] ->
         [[operation, transform_filters(filters, transformer)]]
@@ -160,22 +161,22 @@ defmodule Plausible.Stats.Filters do
     end
   end
 
-  defp traverse(filters, root \\ nil, depth \\ -1) do
+  defp traverse(filters, depth \\ -1) do
     filters
-    |> Enum.flat_map(&traverse_tree(&1, root || &1, depth + 1))
+    |> Enum.flat_map(&traverse_tree(&1, depth + 1))
   end
 
-  defp traverse_tree(filter, root, depth) do
+  defp traverse_tree(filter, depth) do
     case filter do
       [operation, child_filter] when operation in [:not, :ignore_in_totals_query] ->
-        traverse_tree(child_filter, root, depth + 1)
+        traverse_tree(child_filter, depth + 1)
 
       [operation, filters] when operation in [:and, :or] ->
-        traverse(filters, root || filter, depth + 1)
+        traverse(filters, depth + 1)
 
       # Leaf node
       _ ->
-        [{filter, root, depth}]
+        [{filter, depth}]
     end
   end
 end

test/plausible_web/controllers/api/stats_controller/pages_test.exs

@@ -1383,6 +1383,74 @@ defmodule PlausibleWeb.Api.StatsController.PagesTest do
                }
              ]
     end
+
+    test "can compare with previous period", %{conn: conn, site: site} do
+      populate_stats(site, [
+        build(:pageview,
+          pathname: "/page1",
+          timestamp: ~N[2021-01-01 00:00:00]
+        ),
+        build(:pageview,
+          pathname: "/page1",
+          timestamp: ~N[2021-01-02 00:00:00]
+        ),
+        build(:pageview,
+          pathname: "/page2",
+          timestamp: ~N[2021-01-02 00:00:00]
+        ),
+        build(:pageview,
+          pathname: "/page2",
+          timestamp: ~N[2021-01-02 00:00:00]
+        )
+      ])
+
+      conn =
+        get(
+          conn,
+          "/api/stats/#{site.domain}/pages?period=day&date=2021-01-02&comparison=previous_period&detailed=true"
+        )
+
+      assert json_response(conn, 200)["results"] == [
+               %{
+                 "bounce_rate" => 100,
+                 "comparison" => %{
+                   "bounce_rate" => 0,
+                   "pageviews" => 0,
+                   "time_on_page" => 0,
+                   "visitors" => 0,
+                   "change" => %{
+                     "bounce_rate" => nil,
+                     "pageviews" => 100,
+                     "time_on_page" => nil,
+                     "visitors" => 100
+                   }
+                 },
+                 "name" => "/page2",
+                 "pageviews" => 2,
+                 "time_on_page" => nil,
+                 "visitors" => 2
+               },
+               %{
+                 "bounce_rate" => 100,
+                 "name" => "/page1",
+                 "pageviews" => 1,
+                 "time_on_page" => nil,
+                 "visitors" => 1,
+                 "comparison" => %{
+                   "bounce_rate" => 100,
+                   "pageviews" => 1,
+                   "time_on_page" => nil,
+                   "visitors" => 1,
+                   "change" => %{
+                     "bounce_rate" => 0,
+                     "pageviews" => 0,
+                     "time_on_page" => nil,
+                     "visitors" => 0
+                   }
+                 }
+               }
+             ]
+    end
   end
 
   describe "GET /api/stats/:domain/entry-pages" do