title: Rails: select records with maximum date
body: <p>In my app users can save sales reports for given dates. What I want to do now is to query the database and select only the latest sales reports (all those reports that have the maximum date in my table).</p>
  
  <p>I know how to sort all reports by date and to select the one with the highest date - however I don't know how to retrieve multiple reports with the highest date.</p>
  
  <p>How can I achieve that? I'm using Postgres.</p>