Suggest squash commit messages based on recent commits

This commit loads a safe number of commits in order to
calculate squash commit message

app/models/merge_request.rb

@@ -1066,7 +1066,7 @@ class MergeRequest < ApplicationRecord
   # Returns the oldest multi-line commit message, or the MR title if none found
   def default_squash_commit_message
     strong_memoize(:default_squash_commit_message) do
-      commits.without_merge_commits.reverse.find(&:description?)&.safe_message || title
+      recent_commits.without_merge_commits.reverse_each.find(&:description?)&.safe_message || title
     end
   end
 

app/serializers/merge_request_poll_cached_widget_entity.rb

@@ -15,7 +15,7 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
   expose :target_project_id
   expose :squash
   expose :rebase_in_progress?, as: :rebase_in_progress
-  expose :default_squash_commit_message
+  expose :default_squash_commit_message, if: -> (merge_request, _) { merge_request.mergeable? }
   expose :commits_count
   expose :merge_ongoing?, as: :merge_ongoing
   expose :work_in_progress?, as: :work_in_progress
@@ -25,8 +25,9 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
   expose :source_branch_exists?, as: :source_branch_exists
   expose :branch_missing?, as: :branch_missing
 
-  expose :commits_without_merge_commits, using: MergeRequestWidgetCommitEntity do |merge_request|
-    merge_request.commits.without_merge_commits
+  expose :commits_without_merge_commits, using: MergeRequestWidgetCommitEntity,
+    if: -> (merge_request, _) { merge_request.mergeable? } do |merge_request|
+    merge_request.recent_commits.without_merge_commits
   end
   expose :diff_head_sha do |merge_request|
     merge_request.diff_head_sha.presence

changelogs/unreleased/id-mr-squash-message-performance.yml

+---
+title: Suggest squash commit messages based on recent commits
+merge_request: 20231
+author:
+type: performance

spec/models/merge_request_spec.rb

@@ -115,8 +115,24 @@ describe MergeRequest do
     let(:multiline_commits) { subject.commits.select(&is_multiline) }
     let(:singleline_commits) { subject.commits.reject(&is_multiline) }
 
-    it 'returns the oldest multiline commit message' do
-      expect(subject.default_squash_commit_message).to eq(multiline_commits.last.message)
+    context 'when the total number of commits is safe' do
+      it 'returns the oldest multiline commit message' do
+        expect(subject.default_squash_commit_message).to eq(multiline_commits.last.message)
+      end
+    end
+
+    context 'when the total number of commits is big' do
+      let(:safe_number) { 20 }
+
+      before do
+        stub_const('MergeRequestDiff::COMMITS_SAFE_SIZE', safe_number)
+      end
+
+      it 'returns the oldest multiline commit message from safe number of commits' do
+        expect(subject.default_squash_commit_message).to eq(
+          "remove emtpy file.(beacase git ignore empty file)\nadd whitespace test file.\n"
+        )
+      end
     end
 
     it 'returns the merge request title if there are no multiline commits' do

spec/serializers/merge_request_widget_entity_spec.rb

@@ -212,9 +212,29 @@ describe MergeRequestWidgetEntity do
       .to eq(resource.default_merge_commit_message(include_description: true))
   end
 
-  it 'has default_squash_commit_message' do
-    expect(subject[:default_squash_commit_message])
-      .to eq(resource.default_squash_commit_message)
+  describe 'attributes for squash commit message' do
+    context 'when merge request is mergeable' do
+      before do
+        stub_const('MergeRequestDiff::COMMITS_SAFE_SIZE', 20)
+      end
+
+      it 'has default_squash_commit_message and commits_without_merge_commits' do
+        expect(subject[:default_squash_commit_message])
+          .to eq(resource.default_squash_commit_message)
+        expect(subject[:commits_without_merge_commits].size).to eq(12)
+      end
+    end
+
+    context 'when merge request is not mergeable' do
+      before do
+        allow(resource).to receive(:mergeable?).and_return(false)
+      end
+
+      it 'does not have default_squash_commit_message and commits_without_merge_commits' do
+        expect(subject[:default_squash_commit_message]).to eq(nil)
+        expect(subject[:commits_without_merge_commits]).to eq(nil)
+      end
+    end
   end
 
   describe 'new_blob_path' do