Fix breakdown pagination when filtering by goals (#3251)

* Add a failing test

* Bugfix: breakdown by goal pagination

* Update changelog

* Apply comment about comment

CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to this project will be documented in this file.
 - Fixed [IPv6 problems](https://github.com/plausible/analytics/issues/3173) in data migration plausible/analytics#3179
 - Fixed [long URLs display](https://github.com/plausible/analytics/issues/3158) in Outbound Link breakdown view
 - Fixed [Sentry reports](https://github.com/plausible/analytics/discussions/3166) for ingestion requests plausible/analytics#3182
+- Fix breakdown pagination bug in the dashboard details view when filtering by goals
 
 ## v2.0.0 - 2023-07-12
 

lib/plausible_web/controllers/api/stats_controller.ex

@@ -1399,6 +1399,11 @@ defmodule PlausibleWeb.Api.StatsController do
       |> Query.put_filter(filter_name, {:member, items})
       |> Query.remove_event_filters([:goal, :props])
 
+    # Here, we're always only interested in the first page of results 
+    # - the :member filter makes sure that the results always match with 
+    # the items in the given breakdown_results list
+    pagination = {elem(pagination, 0), 1}
+
     res_without_goal =
       Stats.breakdown(site, query_without_goal, filter_name, [:visitors], pagination)
 

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

@@ -1049,6 +1049,46 @@ defmodule PlausibleWeb.Api.StatsController.PagesTest do
              ]
     end
 
+    test "bugfix: pagination on /pages filtered by goal", %{conn: conn, site: site} do
+      populate_stats(
+        site,
+        for i <- 1..30 do
+          build(:event,
+            user_id: i,
+            name: "Signup",
+            pathname: "/signup/#{String.pad_leading(to_string(i), 2, "0")}",
+            timestamp: ~N[2021-01-01 00:01:00]
+          )
+        end
+      )
+
+      request = fn conn, opts ->
+        page = Keyword.fetch!(opts, :page)
+        limit = Keyword.fetch!(opts, :limit)
+        filters = Jason.encode!(%{"goal" => "Signup"})
+
+        conn
+        |> get(
+          "/api/stats/#{site.domain}/pages?date=2021-01-01&period=day&filters=#{filters}&limit=#{limit}&page=#{page}"
+        )
+        |> json_response(200)
+        |> Enum.map(fn %{"name" => "/signup/" <> seq} ->
+          seq
+        end)
+      end
+
+      assert List.first(request.(conn, page: 1, limit: 100)) == "01"
+      assert List.last(request.(conn, page: 1, limit: 100)) == "30"
+      assert List.last(request.(conn, page: 1, limit: 29)) == "29"
+      assert ["01", "02"] = request.(conn, page: 1, limit: 2)
+      assert ["03", "04"] = request.(conn, page: 2, limit: 2)
+      assert ["01", "02", "03", "04", "05"] = request.(conn, page: 1, limit: 5)
+      assert ["06", "07", "08", "09", "10"] = request.(conn, page: 2, limit: 5)
+      assert ["11", "12", "13", "14", "15"] = request.(conn, page: 3, limit: 5)
+      assert ["20"] = request.(conn, page: 20, limit: 1)
+      assert [] = request.(conn, page: 31, limit: 1)
+    end
+
     test "calculates conversion_rate when filtering for goal", %{conn: conn, site: site} do
       populate_stats(site, [
         build(:pageview,