title: Lock model for reading
body: <p>In an <code>ActiveRecord</code> transaction, I need some way to lock the record/table for reading, so the user was not able to select from this table, while the transaction being executed.</p>
  
  <p>Basically, I need something like this</p>
  
  <pre><code>System.transaction do
    s = System.first
    # Something to lock the table or selected record for reading
    s.update_attributes(some_params)
  end
  </code></pre>
  
  <p>Does anyone have an idea on how to do that?</p>