Backfill NotifyLogEntry purpose column using data migrate gem

Taking the opportunity to backfill a column using this gem instead
of having to do it manually.

Author: jhenderson

app/lib/data_migration/backfill_notify_log_entries.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+class BackfillPurposeForNotifyLogEntries < ActiveRecord::Migration[8.1]
+  TEMPLATE_NAME_BY_TEMPLATE_ID = {
+    **GOVUK_NOTIFY_UNUSED_TEMPLATES,
+    **GOVUK_NOTIFY_EMAIL_TEMPLATES.invert,
+    **GOVUK_NOTIFY_SMS_TEMPLATES.invert
+  }.freeze
+
+  def up
+    migration = self.class.name
+    started_at = Time.zone.now
+
+    batch_size = 1000
+    total_records = NotifyLogEntry.count
+    total_batches = (total_records / batch_size.to_f).ceil
+
+    batch_processed = 0
+    records_updated = 0
+
+    # Helps us monitor progress in CloudWatch
+    Rails.logger.info(event: "data_migration_start", migration:, total_records:, total_batches:)
+
+    NotifyLogEntry.find_in_batches(batch_size:) do |notify_log_entries|
+      notify_log_entries.filter_map do |notify_log_entry|
+        template_name =
+          TEMPLATE_NAME_BY_TEMPLATE_ID.fetch(notify_log_entry.template_id, nil)
+
+        next unless template_name
+
+        purpose = NotifyLogEntry.purpose_for_template_name(template_name)
+
+        next unless purpose
+
+        notify_log_entry.update_column(
+          :purpose,
+          NotifyLogEntry.purposes.fetch(purpose)
+        )
+
+        records_updated += 1
+      end
+
+      batch_processed += 1
+
+      Rails.logger.info(
+        event: "data_migration_batch",
+        migration:,
+        records_updated:,
+        batch_size:,
+        batch_processed:,
+        total_batches:,
+      )
+    end
+
+    duration_minutes =  ((Time.zone.now - started_at) / 60.0).round
+
+    Rails.logger.info(
+      event: "data_migration_finish",
+      migration:,
+      duration_minutes:,
+      total_records:,
+      records_updated:,
+    )
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/data/20260304173751_backfill_purpose_for_notify_log_entries.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+DataMigrate::Data.define(version: 20_260_304_173_751)