outside_transaction

Makes sure the given block is only executed when not inside a transaction. If there is an open transaction it will be executed after the next successful commit or not executed at all if there is a rollback or failure.

This is useful when you're notifying another process. If the other process starts processing the message before the transaction is actually committed, it will not see your uncommitted changes as transactions are isolated (usually).

In the example below, if there is a lot of work after saving and article, and ArticleEmailSender is started too soon, it will not find the article you just inserted.

Example:

class Article
  after_create :send_emails

  def send_emails
    outside_transaction do
      Resque.enqueue(ArticleEmailSender, id) # notifies another process
    end
  end
end

This example is using Resque.