ruby-on-rails - 如何避免 nil 对象的错误

Question

记录 id 116 不存在，因此它应该返回 nil 给@conversation。
我试图让它在它为 nil 时重定向，但是当我访问 example.com/messages/show?id=116 时它仍然显示错误。

错误是

undefined method `is_participant?' for nil:NilClass

我肯定看到“is_participant”方法存在于
/usr/local/lib/ruby/gems/1.9.1/gems/mailboxer-0.7.0/app/models/conversation.rb

消息 Controller .rb

def show
  @conversation = Conversation.find_by_id(params[:id])

  unless @conversation.is_participant?(current_user)
    flash[:alert] = "You do not have permission to view that conversation."
    redirect_to :controller => 'messages', :action => 'received'
  end

  @messages = Message.find_by_id(params[:id])
  current_user.read(@conversation)    
end

Answer 1

在调用方法之前，您需要检查 @conversation 是否为 nil。尝试

unless @conversation.present? && @conversation.is_participant?(current_user)