Expose merge train information

Exposed data will be rendered in merge request widgets

ee/app/models/merge_train.rb

@@ -14,6 +14,10 @@ class MergeTrain < ApplicationRecord
       all_in_train(merge_request).first
     end
 
+    def total_count_in_train(merge_request)
+      all_in_train(merge_request).count
+    end
+
     def joined_merge_requests(merge_request)
       MergeRequest.joins(:merge_train)
         .where('merge_requests.target_project_id = ?', merge_request.target_project_id)
@@ -25,6 +29,10 @@ class MergeTrain < ApplicationRecord
     self.class.all_in_train(merge_request).where('merge_trains.id > ?', id)
   end
 
+  def index
+    self.class.all_in_train(merge_request).where('merge_trains.id < ?', id).count
+  end
+
   def first_in_train?
     !follower_in_train?
   end

ee/app/serializers/ee/merge_request_widget_entity.rb

@@ -136,7 +136,15 @@ module EE
       end
 
       expose :merge_trains_enabled?, as: :merge_trains_enabled do |merge_request|
-        merge_request.target_project.merge_trains_enabled?
+        merge_trains_enabled?
+      end
+
+      expose :merge_trains_count, if: -> (*) { merge_trains_enabled? } do |merge_request|
+        MergeTrain.total_count_in_train(merge_request)
+      end
+
+      expose :merge_train_index, if: -> (merge_request) { merge_request.on_train? } do |merge_request|
+        merge_request.merge_train.index
       end
 
       expose :can_push_to_source_branch do |merge_request|
@@ -176,5 +184,9 @@ module EE
       object.base_pipeline&.present(current_user: current_user)
         &.downloadable_path_for_report_type(file_type)
     end
+
+    def merge_trains_enabled?
+      object.target_project.merge_trains_enabled?
+    end
   end
 end

ee/spec/models/merge_train_spec.rb

@@ -63,6 +63,32 @@ describe MergeTrain do
     end
   end
 
+  describe '.total_count_in_train' do
+    subject { described_class.total_count_in_train(merge_request) }
+
+    let!(:merge_request) { create_merge_request_on_train }
+
+    it 'returns the merge request' do
+      is_expected.to eq(1)
+    end
+
+    context 'when the other merge request is on the merge train' do
+      let!(:merge_request_2) { create_merge_request_on_train(source_branch: 'improve/awesome') }
+
+      it 'returns the merge request' do
+        is_expected.to eq(2)
+      end
+    end
+
+    context 'when the merge request is not on merge train' do
+      let(:merge_request) { create(:merge_request) }
+
+      it 'returns empty array' do
+        is_expected.to be(0)
+      end
+    end
+  end
+
   describe '#all_next' do
     subject { merge_train.all_next }
 
@@ -114,6 +140,22 @@ describe MergeTrain do
     end
   end
 
+  describe '#index' do
+    subject { merge_train.index }
+
+    let(:merge_train) { merge_request.merge_train }
+    let!(:merge_request) { create_merge_request_on_train }
+
+    it { is_expected.to eq(0) }
+
+    context 'when the merge train is at the second queue' do
+      let(:merge_train) { merge_request_2.merge_train }
+      let!(:merge_request_2) { create_merge_request_on_train(source_branch: 'improve/awesome') }
+
+      it { is_expected.to eq(1) }
+    end
+  end
+
   def create_merge_request_on_train(target_project: project, target_branch: 'master', source_project: project, source_branch: 'feature')
     create(:merge_request,
       :on_train,

ee/spec/serializers/merge_request_widget_entity_spec.rb

@@ -195,7 +195,36 @@ describe MergeRequestWidgetEntity do
     expect(subject.as_json).to include(:pipeline_id)
   end
 
-  it 'has merge trains flag' do
-    expect(subject.as_json).to include(:merge_trains_enabled)
+  describe 'Merge Trains' do
+    let!(:merge_train) { create(:merge_train, merge_request: merge_request) }
+
+    before do
+      stub_licensed_features(merge_pipelines: true, merge_trains: true)
+      project.update!(merge_trains_enabled: true, merge_pipelines_enabled: true)
+    end
+
+    it 'has merge train entity' do
+      expect(subject.as_json).to include(:merge_trains_enabled)
+      expect(subject.as_json).to include(:merge_trains_count)
+      expect(subject.as_json).to include(:merge_train_index)
+    end
+
+    context 'when the merge train feature is disabled' do
+      before do
+        project.update!(merge_trains_enabled: false)
+      end
+
+      it 'does not have merge trains count' do
+        expect(subject.as_json).not_to include(:merge_trains_count)
+      end
+    end
+
+    context 'when the merge request is not on a merge train' do
+      let!(:merge_train) { }
+
+      it 'does not have merge train index' do
+        expect(subject.as_json).not_to include(:merge_train_index)
+      end
+    end
   end
 end