PERF: Eager load linkables when loading custom sidebar sections (#20490)

Not eager loading was resulting in the N+1 queries problem when
serializing with the `CurrentUserSerializer` as
`CurrentUserSerializer#sidebar_sections` serializes the sections with
`SidebarSectionSerializer` which fetches all the `SidebarUrl` records
for each `SidebarSection` record.

app/serializers/current_user_serializer.rb

@@ -79,6 +79,7 @@ class CurrentUserSerializer < BasicUserSerializer
 
   def sidebar_sections
     SidebarSection
+      .includes(sidebar_section_links: :linkable)
       .where("public OR user_id = ?", object.id)
       .order("(public IS TRUE) DESC")
       .map { |section| SidebarSectionSerializer.new(section, root: false) }

spec/fabricators/sidebar_section_link_fabricator.rb

@@ -7,3 +7,8 @@ Fabricator(:category_sidebar_section_link, from: :sidebar_section_link) do
 end
 
 Fabricator(:tag_sidebar_section_link, from: :sidebar_section_link) { linkable(fabricator: :tag) }
+
+Fabricator(:custom_sidebar_section_link, from: :sidebar_section_link) do
+  linkable(fabricator: :sidebar_url)
+  sidebar_section(fabricator: :sidebar_section)
+end

spec/serializers/current_user_serializer_spec.rb

@@ -297,4 +297,53 @@ RSpec.describe CurrentUserSerializer do
   end
 
   include_examples "User Sidebar Serializer Attributes", described_class
+
+  describe "#sidebar_sections" do
+    fab!(:group) { Fabricate(:group) }
+    fab!(:sidebar_section) { Fabricate(:sidebar_section, user: user) }
+
+    before do
+      group.add(user)
+      SiteSetting.enable_custom_sidebar_sections = group.id
+    end
+
+    it "eager loads sidebar_urls" do
+      custom_sidebar_section_link_1 =
+        Fabricate(:custom_sidebar_section_link, user: user, sidebar_section: sidebar_section)
+
+      # warmup
+      described_class.new(user, scope: Guardian.new(user), root: false).as_json
+
+      initial_count =
+        track_sql_queries do
+          serialized = described_class.new(user, scope: Guardian.new(user), root: false).as_json
+
+          expect(serialized[:sidebar_sections].map { |sidebar_section| sidebar_section.id }).to eq(
+            [sidebar_section.id],
+          )
+
+          expect(serialized[:sidebar_sections].first.links.map { |link| link.id }).to eq(
+            [custom_sidebar_section_link_1.linkable.id],
+          )
+        end.count
+
+      custom_sidebar_section_link_2 =
+        Fabricate(:custom_sidebar_section_link, user: user, sidebar_section: sidebar_section)
+
+      final_count =
+        track_sql_queries do
+          serialized = described_class.new(user, scope: Guardian.new(user), root: false).as_json
+
+          expect(serialized[:sidebar_sections].map { |sidebar_section| sidebar_section.id }).to eq(
+            [sidebar_section.id],
+          )
+
+          expect(serialized[:sidebar_sections].first.links.map { |link| link.id }).to eq(
+            [custom_sidebar_section_link_1.linkable.id, custom_sidebar_section_link_2.linkable.id],
+          )
+        end.count
+
+      expect(initial_count).to eq(final_count)
+    end
+  end
 end