Migrate legacy uploads out of deprecated paths

db/post_migrate/20200409211607_migrate_legacy_attachments.rb

+# frozen_string_literal: true
+
+class MigrateLegacyAttachments < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+
+  MIGRATION = 'LegacyUploadsMigrator'.freeze
+  BATCH_SIZE = 5000
+  DELAY_INTERVAL = 5.minutes.to_i
+
+  class Upload < ActiveRecord::Base
+    self.table_name = 'uploads'
+
+    include ::EachBatch
+  end
+
+  def up
+    Upload.where(uploader: 'AttachmentUploader', model_type: 'Note').each_batch(of: BATCH_SIZE) do |relation, index|
+      start_id, end_id = relation.pluck('MIN(id), MAX(id)').first
+      delay = index * delay_interval
+
+      BackgroundMigrationWorker.perform_in(delay, migration, [start_id, end_id])
+    end
+  end
+
+  def down
+    # no-op
+  end
+end

db/structure.sql

@@ -13147,5 +13147,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200408110856
 20200408153842
 20200408175424
+20200409211607
 \.