From https://once.com/writebook

app/controllers/first_runs_controller.rb

@@ -0,0 +1,25 @@
+class FirstRunsController < ApplicationController
+  allow_unauthenticated_access
+
+  before_action :prevent_running_after_setup
+
+  def show
+    @user = User.new
+  end
+
+  def create
+    user = FirstRun.create!(user_params)
+    start_new_session_for user
+
+    redirect_to root_url
+  end
+
+  private
+    def prevent_running_after_setup
+      redirect_to root_url if User.any?
+    end
+
+    def user_params
+      params.require(:user).permit(:name, :email_address, :password)
+    end
+end