修复邀请码会变的问题

app/models/course.rb

@@ -470,6 +470,7 @@ class Course < ActiveRecord::Base
     if !code || code.size <5
       code = CODES.sample(5).join
       return generate_invite_code if Course.where(invite_code: code).present?
+      self[:invite_code] = code
       save! && reload
     end
     code