Require valid invitation to start sign-up process

app/controllers/signup_controller.rb

@@ -0,0 +1,33 @@
+class SignupController < ApplicationController
+  before_action :require_user_signed_out
+  before_action :require_invitation
+  before_action :set_invitation
+
+  layout "signup"
+
+  def index
+    @invited_by_name = @invitation.user.address
+  end
+
+  private
+
+  def require_invitation
+    if session[:invitation_token].blank?
+      flash.now[:alert] = "You need an invitation to sign up for an account."
+      http_status :unauthorized
+    elsif !valid_invitation?(session[:invitation_token])
+      flash.now[:alert] = "This invitation either doesn't exist or has already been used."
+      http_status :unauthorized
+    end
+
+    @invitation = Invitation.find_by(token: session[:invitation_token])
+  end
+
+  def valid_invitation?(token)
+    Invitation.where(token: session[:invitation_token], used_at: nil).exists?
+  end
+
+  def set_invitation
+    @invitation = Invitation.find_by(token: session[:invitation_token])
+  end
+end