AO3-7405 Prevent N+1 queries on tags#feed

app/controllers/tags_controller.rb

@@ -72,19 +72,16 @@ def show
   end
 
   def feed
-    begin
-      @tag = Tag.find(params[:id])
-    rescue ActiveRecord::RecordNotFound
-      raise ActiveRecord::RecordNotFound, "Couldn't find tag with id '#{params[:id]}'"
-    end
+    @tag = Tag.find(params[:id])
     @tag = @tag.merger if !@tag.canonical? && @tag.merger
     # Temp for testing
     if %w(Fandom Character Relationship).include?(@tag.type.to_s) || @tag.name == 'F/F'
-      if @tag.canonical?
-        @works = @tag.filtered_works.visible_to_all.order('created_at DESC').limit(25)
-      else
-        @works = @tag.works.visible_to_all.order('created_at DESC').limit(25)
-      end
+      @works = if @tag.canonical?
+                 @tag.filtered_works
+               else
+                 @tag.works
+               end
+      @works = @works.visible_to_all.order(created_at: :desc).limit(25).includes(:tags, :language, :series)
     else
       redirect_to(tag_works_path(tag_id: @tag.to_param)) && return
     end

app/helpers/works_helper.rb

@@ -174,7 +174,7 @@ def feed_summary(work)
     text = +"<p>by #{byline(work, { visibility: 'public', only_path: false })}</p>"
     text << work.summary if work.summary
     text << "<p>Words: #{work.word_count}, Chapters: #{chapter_total_display(work)}, Language: #{work.language ? work.language.name : 'English'}</p>"
-    text << "<p>Series: #{series_list_with_work_position(work)}</p>" unless work.series.count.zero?
+    text << "<p>Series: #{series_list_with_work_position(work)}</p>" unless work.series.size.zero?
     # Create list of tags
     text << "<ul>"
     %w(Fandom Rating ArchiveWarning Category Character Relationship Freeform).each do |type|

app/views/tags/feed.atom.builder

@@ -1,10 +1,11 @@
 atom_feed do |feed|
   feed.title "AO3 works tagged '#{@tag.name}'"
-  feed.updated @works.first.created_at if @works.respond_to?(:first) && @works.first.present?
 
-  @works.each do |work|
+  @works.each_with_index do |work, index|
     next if work.unrevealed?
 
+    feed.updated work.created_at if index.zero?
+
     feed.entry work do |entry|
       entry.title work.title
       entry.summary feed_summary(work), type: "html"

features/other_b/subscriptions_fandoms.feature

@@ -17,13 +17,17 @@ Feature: Subscriptions
   Scenario: Subscribe to a test fandom when there are works in it
 
   When I am logged in as "author"
+    And time is frozen at 2026-04-11 16:00
     And I post a work "My Work Title" with category "F/F"
+    And time is frozen at 2026-04-11 17:00
+    And I post a work "Silly Games" with category "F/F"
   When I am logged in as "reader"
     And I view the "F/F" works index
   Then I should see "RSS Feed"
   When I follow "RSS Feed"
   Then I should see "My Work Title"
     And I should see "Stargate SG-1"
+    And the feed updated date should be should be the created date of "Silly Games"
 
   Scenario: Subscribe to a non-test fandom
 

features/step_definitions/subscription_steps.rb

@@ -37,6 +37,11 @@
   expect(page).to have_selector(:xpath, "//author").exactly(int)
 end
 
+Then "the feed updated date should be should be the created date of {string}" do |work|
+  w = Work.find_by(title: work)
+  expect(find(:xpath, "//feed/updated")).to have_content(w.created_at.xmlschema)
+end
+
 # rubocop:disable Cucumber/RegexStepName
 Then /^the (\d+)(?:st|nd|rd|th) feed author should contain "([^"]*)"$/ do |n, text|
   within(:xpath, "//author[#{n}]") do

spec/requests/tags_n_plus_one_spec.rb

@@ -0,0 +1,47 @@
+require "spec_helper"
+
+describe "n+1 queries in the tags controller" do
+  describe "#feed", n_plus_one: true do
+    context "when creating a tag's feed" do
+      let!(:tag) { create(:canonical_fandom, name: "Hermitcraft SMP") }
+
+      subject do
+        proc do
+          get feed_tag_path({ id: tag.id, format: :atom })
+        end
+      end
+
+      context "when all bylines are cached" do
+        populate do |n|
+          create_list(:work, n, fandom_string: "Hermitcraft SMP")
+          subject.call # this caches the bylines
+        end
+
+        it "produces a constant number of queries" do
+          expect do
+            subject.call
+            expect(response.body.scan("<author>").size).to eq(current_scale.to_i)
+          end.to perform_constant_number_of_queries
+        end
+      end
+
+      context "when nothing is cached" do
+        populate do |n|
+          create_list(:work, n, fandom_string: "Hermitcraft SMP")
+        end
+
+        it "performs around 4 queries per work" do
+          # TODO: Ideally, we'd like the uncached tag feed to also have a
+          # constant number of queries, instead of the linear number of queries
+          # we're checking for here. But we also don't want to put too much
+          # unnecessary load on the databases when we have a bunch of cache hits,
+          # so this requires some care & tuning.
+          expect do
+            subject.call
+            expect(response.body.scan("<author>").size).to eq(current_scale.to_i)
+          end.to perform_linear_number_of_queries(slope: 4).with_warming_up
+        end
+      end
+    end
+  end
+end