Backfill NotifyLogEntry purpose for existing records

Add a data migration to populate `notify_log_entries.purpose` from the template
ID→name mapping. The mapping includes both current template IDs and historical
IDs (recovered from git history) so older records can be backfilled accurately.

Jira-Issue: MAV-502

Author: jhenderson

app/lib/data_migration/backfill_notify_log_entries.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+TEMPLATE_NAME_BY_TEMPLATE_ID = {
+  **GOVUK_NOTIFY_UNUSED_TEMPLATES,
+  **GOVUK_NOTIFY_EMAIL_TEMPLATES.invert,
+  **GOVUK_NOTIFY_SMS_TEMPLATES.invert
+}.freeze
+
+class DataMigration::BackfillNotifyLogEntries
+  def call
+    progress_bar =
+      # rubocop:disable Rails/SaveBang
+      ProgressBar.create(
+        total: NotifyLogEntry.count,
+        format: "%a %b\u{15E7}%i %p%% %t",
+        progress_mark: " ",
+        remainder_mark: "\u{FF65}"
+      )
+    # rubocop:enable Rails/SaveBang
+
+    NotifyLogEntry.find_in_batches 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)
+        )
+      end
+
+      progress_bar.progress += 1
+    end
+
+    progress_bar.finish
+  end
+
+  def self.call(...) = new(...).call
+
+  private_class_method :new
+end

lib/tasks/data_migration.rake

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+namespace :data_migration do
+  desc "Backfill purpose column for NotifyLogEntry records"
+  task backfill_notify_log_entries: :environment do
+    DataMigration::BackfillNotifyLogEntries.call
+  end
+end

spec/lib/data_migration/backfill_notify_log_entries_spec.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+describe DataMigration::BackfillNotifyLogEntries do
+  describe ".call" do
+    let!(:mapped_entry) do
+      create(
+        :notify_log_entry,
+        :email,
+        template_id: "14e88a09-4281-4257-9574-6afeaeb42715",
+        purpose: nil
+      )
+    end
+    let!(:unknown_entry) do
+      create(
+        :notify_log_entry,
+        :email,
+        template_id: "99999999-9999-9999-9999-999999999999",
+        purpose: nil
+      )
+    end
+
+    it "backfills purpose from historical template ID mappings" do
+      described_class.call
+
+      expect(mapped_entry.reload.purpose).to eq("consent_request")
+      expect(unknown_entry.reload.purpose).to be_nil
+    end
+  end
+end