1 files changed, 80 insertions, 0 deletions
diff --git a/app/services/milestones/promote_service.rb b/app/services/milestones/promote_service.rb
new file mode 100644
index 00000000000..bd9cfd4e0ea
--- /dev/null
+++ b/app/services/milestones/promote_service.rb
@@ -0,0 +1,80 @@
+module Milestones
+  class PromoteService < Milestones::BaseService
+    PromoteMilestoneError = Class.new(StandardError)
+
+    def execute(milestone)
+      check_project_milestone!(milestone)
+
+      Milestone.transaction do
+        # Destroy all milestones with same title across projects
+        destroy_old_milestones(milestone)
+
+        group_milestone = clone_project_milestone(milestone)
+
+        move_children_to_group_milestone(group_milestone)
+
+        # Just to be safe
+        unless group_milestone.valid?
+          raise_error(group_milestone.errors.full_messages.to_sentence)
+        end
+
+        group_milestone
+      end
+    end
+
+    private
+
+    def milestone_ids_for_merge(group_milestone)
+      # Pluck need to be used here instead of select so the array of ids
+      # is persistent after old milestones gets deleted.
+      @milestone_ids_for_merge ||= begin
+        search_params = { title: group_milestone.title, project_ids: group_project_ids, state: 'all' }
+        milestones = MilestonesFinder.new(search_params).execute
+        milestones.pluck(:id)
+      end
+    end
+
+    def move_children_to_group_milestone(group_milestone)
+      milestone_ids_for_merge(group_milestone).in_groups_of(100) do |milestone_ids|
+        update_children(group_milestone, milestone_ids)
+      end
+    end
+
+    def check_project_milestone!(milestone)
+      raise_error('Only project milestones can be promoted.') unless milestone.project_milestone?
+    end
+
+    def clone_project_milestone(milestone)
+      params = milestone.slice(:title, :description, :start_date, :due_date, :state_event)
+
+      create_service = CreateService.new(group, current_user, params)
+
+      create_service.execute
+    end
+
+    def update_children(group_milestone, milestone_ids)
+      issues = Issue.where(project_id: group_project_ids, milestone_id: milestone_ids)
+      merge_requests = MergeRequest.where(source_project_id: group_project_ids, milestone_id: milestone_ids)
+
+      [issues, merge_requests].each do |issuable_collection|
+        issuable_collection.update_all(milestone_id: group_milestone.id)
+      end
+    end
+
+    def group
+      @group ||= parent.group || raise_error('Project does not belong to a group.')
+    end
+
+    def destroy_old_milestones(group_milestone)
+      Milestone.where(id: milestone_ids_for_merge(group_milestone)).destroy_all
+    end
+
+    def group_project_ids
+      @group_project_ids ||= group.projects.map(&:id)
+    end
+
+    def raise_error(message)
+      raise PromoteMilestoneError, "Promotion failed - #{message}"
+    end
+  end
+end