Merge branch 'conditionally-eager-load-event-target-authors' into 'master'

Eager load event target authors whenever possible

Closes #41618

See merge request gitlab-org/gitlab-ce!16199

app/models/event.rb

@@ -48,7 +48,18 @@ class Event < ActiveRecord::Base
 
   belongs_to :author, class_name: "User"
   belongs_to :project
-  belongs_to :target, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
+
+  belongs_to :target, -> {
+    # If the association for "target" defines an "author" association we want to
+    # eager-load this so Banzai & friends don't end up performing N+1 queries to
+    # get the authors of notes, issues, etc.
+    if reflections['events'].active_record.reflect_on_association(:author)
+      includes(:author)
+    else
+      self
+    end
+  }, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
+
   has_one :push_event_payload
 
   # Callbacks

changelogs/unreleased/conditionally-eager-load-event-target-authors.yml

+---
+title: Eager load event target authors whenever possible
+merge_request:
+author:
+type: performance

spec/features/dashboard/activity_spec.rb

@@ -24,6 +24,7 @@ feature 'Dashboard > Activity' do
     end
 
     let(:note) { create(:note, project: project, noteable: merge_request) }
+    let(:milestone) { create(:milestone, :active, project: project, title: '1.0') }
 
     let!(:push_event) do
       event = create(:push_event, project: project, author: user)
@@ -54,6 +55,10 @@ feature 'Dashboard > Activity' do
       create(:event, :commented, project: project, target: note, author: user)
     end
 
+    let!(:milestone_event) do
+      create(:event, :closed, project: project, target: milestone, author: user)
+    end
+
     before do
       project.add_master(user)
 
@@ -68,6 +73,7 @@ feature 'Dashboard > Activity' do
         expect(page).to have_content('accepted')
         expect(page).to have_content('closed')
         expect(page).to have_content('commented on')
+        expect(page).to have_content('closed milestone')
       end
     end
 
@@ -107,6 +113,7 @@ feature 'Dashboard > Activity' do
         expect(page).not_to have_content('accepted')
         expect(page).to have_content('closed')
         expect(page).not_to have_content('commented on')
+        expect(page).to have_content('closed milestone')
       end
     end
 

spec/models/event_spec.rb

@@ -347,6 +347,22 @@ describe Event do
     end
   end
 
+  describe '#target' do
+    it 'eager loads the author of an event target' do
+      create(:closed_issue_event)
+
+      events = described_class.preload(:target).all.to_a
+      count = ActiveRecord::QueryRecorder
+        .new { events.first.target.author }.count
+
+      # This expectation exists to make sure the test doesn't pass when the
+      # author is for some reason not loaded at all.
+      expect(events.first.target.author).to be_an_instance_of(User)
+
+      expect(count).to be_zero
+    end
+  end
+
   def create_push_event(project, user)
     event = create(:push_event, project: project, author: user)