1 files changed, 84 insertions, 0 deletions
diff --git a/app/services/milestones/transfer_service.rb b/app/services/milestones/transfer_service.rb
new file mode 100644
index 00000000000..1efbfed4853
--- /dev/null
+++ b/app/services/milestones/transfer_service.rb
@@ -0,0 +1,84 @@
+# frozen_string_literal: true
+
+# Milestones::TransferService class
+#
+# Used for recreating the missing group milestones at project level when
+# transferring a project to a new namespace
+#
+module Milestones
+  class TransferService
+    attr_reader :current_user, :old_group, :project
+
+    def initialize(current_user, old_group, project)
+      @current_user = current_user
+      @old_group = old_group
+      @project = project
+    end
+
+    def execute
+      return unless old_group.present?
+
+      Milestone.transaction do
+        milestones_to_transfer.find_each do |milestone|
+          new_milestone = find_or_create_milestone(milestone)
+
+          update_issues_milestone(milestone.id, new_milestone&.id)
+          update_merge_requests_milestone(milestone.id, new_milestone&.id)
+        end
+      end
+    end
+
+    private
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def milestones_to_transfer
+      Milestone.from_union([
+          group_milestones_applied_to_issues,
+          group_milestones_applied_to_merge_requests
+        ])
+        .reorder(nil)
+        .distinct
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def group_milestones_applied_to_issues
+      Milestone.joins(:issues)
+        .where(
+          issues: { project_id: project.id },
+          group_id: old_group.id
+        )
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def group_milestones_applied_to_merge_requests
+      Milestone.joins(:merge_requests)
+        .where(
+          merge_requests: { target_project_id: project.id },
+          group_id: old_group.id
+        )
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    def find_or_create_milestone(milestone)
+      params = milestone.attributes.slice('title', 'description', 'start_date', 'due_date')
+
+      FindOrCreateService.new(project, current_user, params).execute
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def update_issues_milestone(old_milestone_id, new_milestone_id)
+      Issue.where(project: project, milestone_id: old_milestone_id)
+        .update_all(milestone_id: new_milestone_id)
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def update_merge_requests_milestone(old_milestone_id, new_milestone_id)
+      MergeRequest.where(project: project, milestone_id: old_milestone_id)
+        .update_all(milestone_id: new_milestone_id)
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+  end
+end