AO3-7350 Fix N+1 queries on readings index

app/controllers/readings_controller.rb

@@ -16,7 +16,7 @@ def index
       @readings = @readings.where(toread: true)
       @page_subtitle = t(".marked_for_later_page_title")
     end
-    @readings = @readings.order("last_viewed DESC")
+    @readings = @readings.order("last_viewed DESC").includes(work: :pseuds)
     @pagy, @readings = pagy(@readings)
   end
 

spec/requests/readings_n_plus_one_spec.rb

@@ -0,0 +1,53 @@
+require "spec_helper"
+
+describe "n+1 queries in the readings controller" do
+  include LoginMacros
+
+  describe "#index", n_plus_one: true do
+    context "when displaying a user's reading history" do
+      let!(:user) { create(:user) }
+
+      subject do
+        proc do
+          get user_readings_path(user)
+        end
+      end
+
+      before do
+        fake_login_known_user(user)
+      end
+
+      context "when all works are cached" do
+        populate do |n|
+          create_list(:reading, n, user: user)
+          subject.call # this caches the blurbs
+        end
+
+        it "produces a constant number of queries" do
+          expect do
+            subject.call
+            expect(response.body.scan('<li id="work_').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(:reading, n, user: user)
+        end
+
+        it "performs around 12 queries per reading" do
+          # TODO: Ideally, we'd like the uncached reading listings 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('<li id="work_').size).to eq(current_scale.to_i)
+          end.to perform_linear_number_of_queries(slope: 12).with_warming_up
+        end
+      end
+    end
+  end
+end