title: Group by multiple columns in ActiveRecord
body: <p>Using Rails 3.0, i'm trying to do a count on number of times a each combination of column1 and column2 occur.  </p>
  
  <p>IE Column A has values A-Z and Column B has values 1-5, i want a count of A1, A2, etc.</p>
  
  <p>Is there a way to either group by multiple columns or join the two columns and group of the result?  In reading the documentation, it wasn't clear how to accomplish this.</p>