Allow getting all paths (old & new) involved in MR

Fetch database columns directly if available

app/models/compare.rb

 # frozen_string_literal: true
+require 'set'
+
 class Compare
   include Gitlab::Utils::StrongMemoize
@@ -77,4 +79,13 @@ class Compare
       head_sha: head_commit_sha
     )
   end
+
+  def modified_paths
+    paths = Set.new
+    diffs.diff_files.each do |diff|
+      paths.add diff.old_path
+      paths.add diff.new_path
+    end
+    paths.to_a
+  end
 end

app/models/merge_request.rb

@@ -409,6 +409,18 @@ class MergeRequest < ActiveRecord::Base
     merge_request_diff&.real_size || diffs.real_size
   end
+  def modified_paths(past_merge_request_diff: nil)
+    diffs = if past_merge_request_diff
+              past_merge_request_diff
+            elsif compare
+              compare
+            else
+              self.merge_request_diff
+            end
+
+    diffs.modified_paths
+  end
+
   def diff_base_commit
     if persisted?
       merge_request_diff.base_commit

app/models/merge_request_diff.rb

@@ -6,6 +6,7 @@ class MergeRequestDiff < ActiveRecord::Base
   include ManualInverseAssociation
   include IgnorableColumn
   include EachBatch
+  include Gitlab::Utils::StrongMemoize
   # Don't display more than 100 commits at once
   COMMITS_SAFE_SIZE = 100
@@ -234,6 +235,12 @@ class MergeRequestDiff < ActiveRecord::Base
   end
   # rubocop: enable CodeReuse/ServiceClass
+  def modified_paths
+    strong_memoize(:modified_paths) do
+      merge_request_diff_files.pluck(:new_path, :old_path).flatten.uniq
+    end
+  end
+
   private
   def create_merge_request_diff_files(diffs)

spec/models/compare_spec.rb

@@ -92,4 +92,33 @@ describe Compare do
       expect(subject.diff_refs.head_sha).to eq(head_commit.id)
     end
   end
+
+  describe '#modified_paths' do
+    context 'changes are present' do
+      let(:raw_compare) do
+        Gitlab::Git::Compare.new(
+          project.repository.raw_repository, 'before-create-delete-modify-move', 'after-create-delete-modify-move'
+        )
+      end
+
+      it 'returns affected file paths, without duplication' do
+        expect(subject.modified_paths).to contain_exactly(*%w{
+          foo/for_move.txt
+          foo/bar/for_move.txt
+          foo/for_create.txt
+          foo/for_delete.txt
+          foo/for_edit.txt
+        })
+      end
+    end
+
+    context 'changes are absent' do
+      let(:start_commit) { sample_commit }
+      let(:head_commit) { sample_commit }
+
+      it 'returns empty array' do
+        expect(subject.modified_paths).to eq([])
+      end
+    end
+  end
 end

spec/models/merge_request_diff_spec.rb

@@ -232,4 +232,17 @@ describe MergeRequestDiff do
       expect(commits.map(&:sha)).to match_array(commit_shas)
     end
   end
+
+  describe '#modified_paths' do
+    subject do
+      diff = create(:merge_request_diff)
+      create(:merge_request_diff_file, :new_file, merge_request_diff: diff)
+      create(:merge_request_diff_file, :renamed_file, merge_request_diff: diff)
+      diff
+    end
+
+    it 'returns affected file paths' do
+      expect(subject.modified_paths).to eq(%w{foo bar baz})
+    end
+  end
 end

spec/models/merge_request_spec.rb

@@ -631,6 +631,44 @@ describe MergeRequest do
     end
   end
+  describe '#modified_paths' do
+    let(:paths) { double(:paths) }
+    subject(:merge_request) { build(:merge_request) }
+
+    before do
+      expect(diff).to receive(:modified_paths).and_return(paths)
+    end
+
+    context 'when past_merge_request_diff is specified' do
+      let(:another_diff) { double(:merge_request_diff) }
+      let(:diff) { another_diff }
+
+      it 'returns affected file paths from specified past_merge_request_diff' do
+        expect(merge_request.modified_paths(past_merge_request_diff: another_diff)).to eq(paths)
+      end
+    end
+
+    context 'when compare is present' do
+      let(:compare) { double(:compare) }
+      let(:diff) { compare }
+
+      it 'returns affected file paths from compare' do
+        merge_request.compare = compare
+
+        expect(merge_request.modified_paths).to eq(paths)
+      end
+    end
+
+    context 'when no arguments provided' do
+      let(:diff) { merge_request.merge_request_diff }
+      subject(:merge_request) { create(:merge_request, source_branch: 'feature', target_branch: 'master') }
+
+      it 'returns affected file paths for merge_request_diff' do
+        expect(merge_request.modified_paths).to eq(paths)
+      end
+    end
+  end
+
   describe "#related_notes" do
     let!(:merge_request) { create(:merge_request) }