Refactor to inline version_index to its only use

Add spec and factory for multiple diffs

app/helpers/merge_requests_helper.rb

@@ -80,12 +80,6 @@ module MergeRequestsHelper
     diffs_project_merge_request_path(project, merge_request, diff_id: merge_request_diff.id, start_sha: start_sha)
   end
 
-  def version_index(merge_request_diff)
-    return if @merge_request_diffs.empty?
-
-    @merge_request_diffs.size - @merge_request_diffs.index(merge_request_diff)
-  end
-
   def merge_params(merge_request)
     {
       auto_merge_strategy: AutoMergeService::STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS,

app/serializers/merge_request_diff_entity.rb

@@ -6,13 +6,13 @@ class MergeRequestDiffEntity < Grape::Entity
   include MergeRequestsHelper
 
   expose :version_index do |merge_request_diff|
-    @merge_request_diffs = options[:merge_request_diffs]
+    merge_request_diffs = options[:merge_request_diffs]
     diff = options[:merge_request_diff]
 
-    next unless @merge_request_diffs.include?(diff)
-    next unless @merge_request_diffs.size > 1
+    next unless merge_request_diffs.include?(diff)
+    next unless merge_request_diffs.size > 1
 
-    version_index(merge_request_diff)
+    merge_request_diffs.size - merge_request_diffs.index(merge_request_diff)
   end
 
   expose :created_at

spec/factories/merge_requests.rb

@@ -341,6 +341,11 @@ FactoryBot.define do
         create(:diff_note_on_merge_request, noteable: mr, project: mr.source_project)
       end
     end
+    factory :merge_request_with_multiple_diffs do
+      after(:create) do |mr|
+        mr.merge_request_diffs.create!(head_commit_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9')
+      end
+    end
 
     factory :labeled_merge_request do
       transient do

spec/serializers/merge_request_diff_entity_spec.rb

@@ -10,7 +10,9 @@ RSpec.describe MergeRequestDiffEntity do
   let(:merge_request_diffs) { merge_request.merge_request_diffs }
   let(:merge_request_diff) { merge_request_diffs.first }
 
-  let(:entity) do
+  let(:entity) { initialize_entity(merge_request, merge_request_diff) }
+
+  def initialize_entity(merge_request, merge_request_diff)
     described_class.new(
       merge_request_diff,
       request: request,
@@ -70,6 +72,22 @@ RSpec.describe MergeRequestDiffEntity do
 
       it_behaves_like 'version_index is nil'
     end
+
+    context 'when @merge_request_diffs.size > 1' do
+      let(:merge_request) { create(:merge_request_with_multiple_diffs) }
+
+      it 'returns difference between size and diff index' do
+        expect(merge_request_diffs.size).to eq(2)
+
+        # diff index: 0
+        subject = initialize_entity(merge_request, merge_request_diffs.first)
+        expect(subject.as_json[:version_index]).to eq(2)
+
+        # diff index: 1
+        subject = initialize_entity(merge_request, merge_request_diffs.last)
+        expect(subject.as_json[:version_index]).to eq(1)
+      end
+    end
   end
 
   describe '#short_commit_sha' do