ruby-on-rails – 这段代码如何抛出ActiveRecord :: ReadOnlyRec

Purchase模型包含以下代码：

def decrement_coupon
  coupon = Coupon.find_by_code(coupon_code,:readonly => false)
  return unless coupon.respond_to?(:uses)
  coupon.uses = coupon.uses - 1
  coupon.save
end

但是前几天我收到了这个错误：

ActiveRecord::ReadOnlyRecord - ActiveRecord::ReadOnlyRecord:
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/persistence.rb:245:in `create_or_update'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/callbacks.rb:273:in `create_or_update'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activesupport-3.0.6/lib/active_support/callbacks.rb:419:in `_run_save_callbacks'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/callbacks.rb:273:in `create_or_update'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/persistence.rb:39:in `save'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/validations.rb:43:in `save'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/attribute_methods/dirty.rb:21:in `save'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:240:in `save'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:292:in `with_transaction_returning_status'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/connection_adapters/abstract/database_statements.rb:139:in `transaction'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:207:in `transaction'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:290:in `with_transaction_returning_status'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:240:in `save'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:251:in `rollback_active_record_state!'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/transactions.rb:239:in `save'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/associations/association_proxy.rb:222:in `send'
 /usr/local/rvm/gems/ree-1.8.7-2010.02@lyconic/gems/activerecord-3.0.6/lib/active_record/associations/association_proxy.rb:222:in `method_missing'
 /var/www/88tactical_production/releases/20110823232510/app/models/purchase.rb:69:in `decrement_coupon'

是：readonly => false不保证非读取记录？将此功能移至优惠券模型会有所作为吗？ (无论如何,我可能会这样做)