Improve the performance for MR discussions loadosw-improve-discussions-query

It considerably improves the query for preloading MR
discussion diffs for:

1. MR diff comments (made at Diffs tab; presented at the main MR page)
2. Comments on commits being listed at the MR

Additionally, it also prevents a second similar query that was
unnecessarily being made.

1 files changed, 30 insertions, 29 deletions

diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 11234982dd4..57bf1cbf933 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -652,9 +652,35 @@ describe MergeRequest do
     end
   end
 
-  describe '#preload_discussions_diff_highlight' do
+  describe '#discussions_diffs' do
     let(:merge_request) { create(:merge_request) }
 
+    shared_examples 'discussions diffs collection' do
+      it 'initializes Gitlab::DiscussionsDiff::FileCollection with correct data' do
+        note_diff_file = diff_note.note_diff_file
+
+        expect(Gitlab::DiscussionsDiff::FileCollection)
+          .to receive(:new)
+          .with([note_diff_file])
+          .and_call_original
+
+        result = merge_request.discussions_diffs
+
+        expect(result).to be_a(Gitlab::DiscussionsDiff::FileCollection)
+      end
+
+      it 'eager loads relations' do
+        result = merge_request.discussions_diffs
+
+        recorder = ActiveRecord::QueryRecorder.new do
+          result.first.diff_note
+          result.first.diff_note.project
+        end
+
+        expect(recorder.count).to be_zero
+      end
+    end
+
     context 'with commit diff note' do
       let(:other_merge_request) { create(:merge_request) }
 
@@ -666,40 +692,15 @@ describe MergeRequest do
         create(:diff_note_on_commit, project: other_merge_request.project)
       end
 
-      it 'preloads diff highlighting' do
-        expect_next_instance_of(Gitlab::DiscussionsDiff::FileCollection) do |collection|
-          note_diff_file = diff_note.note_diff_file
-
-          expect(collection)
-            .to receive(:load_highlight)
-            .with([note_diff_file.id]).and_call_original
-        end
-
-        merge_request.preload_discussions_diff_highlight
-      end
+      it_behaves_like 'discussions diffs collection'
     end
 
     context 'with merge request diff note' do
-      let!(:unresolved_diff_note) do
+      let!(:diff_note) do
         create(:diff_note_on_merge_request, project: merge_request.project, noteable: merge_request)
       end
 
-      let!(:resolved_diff_note) do
-        create(:diff_note_on_merge_request, :resolved, project: merge_request.project, noteable: merge_request)
-      end
-
-      it 'preloads diff highlighting' do
-        expect_next_instance_of(Gitlab::DiscussionsDiff::FileCollection) do |collection|
-          note_diff_file = unresolved_diff_note.note_diff_file
-
-          expect(collection)
-            .to receive(:load_highlight)
-            .with([note_diff_file.id])
-            .and_call_original
-        end
-
-        merge_request.preload_discussions_diff_highlight
-      end
+      it_behaves_like 'discussions diffs collection'
     end
   end