Add Board Milestones to Group export

We want to be able to export the milestones on Boards for groups as
well.

Prior to this change, adding `:milestone` to the export config file
would result in an error when the `serialize_hash` method was called on
the MilestoneStruct, which is used for predefined, system wide
milestones.

Add this method to the struct so that it responds to the needed
interface and add `:milestone` to the config file for export.

app/models/milestone.rb

@@ -3,7 +3,13 @@
 class Milestone < ApplicationRecord
   # Represents a "No Milestone" state used for filtering Issues and Merge
   # Requests that have no milestone assigned.
-  MilestoneStruct = Struct.new(:title, :name, :id)
+  MilestoneStruct = Struct.new(:title, :name, :id) do
+    # Ensure these models match the interface required for exporting
+    def serializable_hash(_opts = {})
+      { title: title, name: name, id: id }
+    end
+  end
+
   None = MilestoneStruct.new('No Milestone', 'No Milestone', 0)
   Any = MilestoneStruct.new('Any Milestone', '', -1)
   Upcoming = MilestoneStruct.new('Upcoming', '#upcoming', -2)

ee/spec/lib/ee/gitlab/import_export/group/tree_saver_spec.rb

@@ -138,6 +138,51 @@ describe Gitlab::ImportExport::Group::TreeSaver do
         expect(assignee_list['user_id']).to eq(user.id)
       end
     end
+
+    context 'when there are boards with predefined milestones' do
+      let(:milestone) { Milestone::Upcoming }
+
+      before do
+        create(:board, group: group, milestone_id: milestone.id)
+      end
+
+      it 'saves the milestone data' do
+        expect_successful_save(group_tree_saver)
+
+        board_data = saved_group_json['boards'].last
+
+        expect(board_data).to include(
+          'milestone_id' => milestone.id,
+          'milestone'    => {
+            'id'    => milestone.id,
+            'name'  => milestone.name,
+            'title' => milestone.title
+          }
+        )
+      end
+    end
+
+    context 'when there are boards with persisted milestones' do
+      let(:milestone) { create(:milestone) }
+
+      before do
+        create(:board, group: group, milestone: milestone)
+      end
+
+      it 'saves the milestone data' do
+        expect_successful_save(group_tree_saver)
+
+        board_data = saved_group_json['boards'].last
+
+        expect(board_data).to include(
+          'milestone_id' => milestone.id,
+          'milestone'    => a_hash_including(
+            'id'    => milestone.id,
+            'title' => milestone.title
+          )
+        )
+      end
+    end
   end
 
   def expect_successful_save(group_tree_saver)

lib/gitlab/import_export/group/import_export.yml

@@ -70,6 +70,7 @@ ee:
             - :push_event_payload
       - boards:
         - :board_assignee
+        - :milestone
         - labels:
           - :priorities
         - lists:

spec/lib/gitlab/import_export/group/tree_saver_spec.rb

@@ -189,7 +189,7 @@ describe Gitlab::ImportExport::Group::TreeSaver do
     create(:group_badge, group: group)
     group_label = create(:group_label, group: group)
     create(:label_priority, label: group_label, priority: 1)
-    board = create(:board, group: group)
+    board = create(:board, group: group, milestone_id: Milestone::Upcoming.id)
     create(:list, board: board, label: group_label)
     create(:group_badge, group: group)
 

spec/models/milestone_spec.rb

@@ -3,6 +3,18 @@
 require 'spec_helper'
 
 describe Milestone do
+  describe 'MilestoneStruct#serializable_hash' do
+    let(:predefined_milestone) { described_class::MilestoneStruct.new('Test Milestone', '#test', 1) }
+
+    it 'presents the predefined milestone as a hash' do
+      expect(predefined_milestone.serializable_hash).to eq(
+        title: predefined_milestone.title,
+        name: predefined_milestone.name,
+        id: predefined_milestone.id
+      )
+    end
+  end
+
   describe 'modules' do
     context 'with a project' do
       it_behaves_like 'AtomicInternalId' do