Add data update script for backfilling usernames (#12728)

lib/data_update_scripts/20210218041143_backfill_usernames.rb

@@ -0,0 +1,9 @@
+module DataUpdateScripts
+  class BackfillUsernames
+    def run
+      User.where(username: nil).find_each do |user|
+        user.update(username: "user#{user.id}")
+      end
+    end
+  end
+end

spec/lib/data_update_scripts/backfill_usernames_spec.rb

@@ -0,0 +1,14 @@
+require "rails_helper"
+require Rails.root.join(
+  "lib/data_update_scripts/20210218041143_backfill_usernames.rb",
+)
+
+describe DataUpdateScripts::BackfillUsernames do
+  it " " do
+    user = create(:user)
+    user.update_column(:username, nil)
+    expect do
+      described_class.new.run
+    end.to change { user.reload.username }.from(nil).to("user#{user.id}")
+  end
+end