feat: use custom migration template with backfill for adding webauthn_id column

Replaces invoke of `active_record:migration` with a migration template
that includes the `webauthn_id` column and backfill for existing records.

Author: santiagorodriguez96

lib/generators/devise/webauthn/webauthn_id/templates/add_webauthn_id.rb.erb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class AddWebauthnIdTo<%= table_name.camelize %> < ActiveRecord::Migration[<%= Rails.version.to_f %>]
+  def up
+    add_column :<%= table_name %>, :webauthn_id, :string
+    add_index :<%= table_name %>, :webauthn_id, unique: true
+
+    # WARNING: The code below backfills webauthn_id for all existing records
+    # one row at a time. For larger tables, consider removing it and running
+    # the backfill separately (e.g., in a background job or maintenance task).
+    #
+    # Worth noting: PostgreSQL and MySQL support single-query backfills:
+    #
+    #   PostgreSQL:
+    #     UPDATE <%= table_name %> SET webauthn_id = encode(gen_random_bytes(64), 'base64') WHERE webauthn_id IS NULL
+    #
+    #   MySQL:
+    #     UPDATE <%= table_name %> SET webauthn_id = TO_BASE64(RANDOM_BYTES(64)) WHERE webauthn_id IS NULL
+    #
+    execute("SELECT id FROM <%= table_name %> WHERE webauthn_id IS NULL").each do |row|
+      webauthn_id = WebAuthn.generate_user_id
+      execute("UPDATE <%= table_name %> SET webauthn_id = #{connection.quote(webauthn_id)} WHERE id = #{row['id']}")
+    end
+  end
+
+  def down
+    remove_column :<%= table_name %>, :webauthn_id
+  end
+end

lib/generators/devise/webauthn/webauthn_id/webauthn_id_generator.rb

@@ -6,17 +6,22 @@
 module Devise
   module Webauthn
     class WebauthnIdGenerator < Rails::Generators::Base
+      include Rails::Generators::Migration
+
       hide!
       namespace "devise:webauthn:webauthn_id"
 
+      source_root File.expand_path("templates", __dir__)
+
       desc "Add webauthn_id field to User model"
       class_option :resource_name, type: :string, default: "user", desc: "The resource name for Devise (default: user)"
 
+      def self.next_migration_number(dirname)
+        ActiveRecord::Generators::Base.next_migration_number(dirname)
+      end
+
       def generate_migration
-        invoke "active_record:migration", [
-          "add_webauthn_id_to_#{user_table_name}",
-          "webauthn_id:string:uniq"
-        ]
+        migration_template "add_webauthn_id.rb.erb", "db/migrate/add_webauthn_id_to_#{table_name}.rb"
       end
 
       def show_instructions
@@ -33,7 +38,7 @@ def show_instructions
 
       private
 
-      def user_table_name
+      def table_name
         options[:resource_name].pluralize.underscore
       end
     end

spec/generators/devise/webauthn/webauthn_id_generator_spec.rb

@@ -9,25 +9,35 @@
 
   before do
     prepare_destination
-    allow(generator_instance).to receive(:invoke)
     invoke generator_instance
   end
 
   context "when using default resource name" do
     let(:generator_instance) { generator }
 
-    it "invokes the active_record:migration generator with correct arguments" do
-      expect(generator).to have_received(:invoke).with("active_record:migration",
-                                                       ["add_webauthn_id_to_users", "webauthn_id:string:uniq"])
+    it "creates a migration that adds webauthn_id to users" do
+      assert_migration "db/migrate/add_webauthn_id_to_users.rb" do |migration|
+        assert_match(/add_column :users, :webauthn_id, :string/, migration)
+        assert_match(/add_index :users, :webauthn_id, unique: true/, migration)
+      end
+    end
+
+    it "creates a migration that backfills existing records" do
+      assert_migration "db/migrate/add_webauthn_id_to_users.rb" do |migration|
+        assert_match(/SELECT id FROM users WHERE webauthn_id IS NULL/, migration)
+        assert_match(/WebAuthn\.generate_user_id/, migration)
+      end
     end
   end
 
   context "when using a custom resource name" do
     let(:generator_instance) { generator([destination_root], ["--resource_name=admin"]) }
 
-    it "invokes the active_record:migration generator with correct arguments" do
-      expect(generator).to have_received(:invoke).with("active_record:migration",
-                                                       ["add_webauthn_id_to_admins", "webauthn_id:string:uniq"])
+    it "creates a migration that adds webauthn_id to admins" do
+      assert_migration "db/migrate/add_webauthn_id_to_admins.rb" do |migration|
+        assert_match(/add_column :admins, :webauthn_id, :string/, migration)
+        assert_match(/add_index :admins, :webauthn_id, unique: true/, migration)
+      end
     end
   end
 end