Implementing Group By and Sum

I want to perfom the sql query,
SELECT employee_id, sum(period) FROM activity_charts group by
employee_id

So far, I was able to do SELECT employee_id, period FROM
activity_charts group by employee_id
My report_controller.rb is
class ReportController < ApplicationController
def index
        @activities = ActivityChart.find(:all, :select =>
'employee_id,period', :group => "employee_id")

end
end

and the index.html.erb is
<h1>Total Hours Worked</h1>
<table>
<tr>
<th width ="180px">Employee</th>
<th width ="60px">Minutes</th>
</tr>
<% @activities.each do |c| %>
  <tr>
    <td><%=h c.employee.first_name %>&nbsp;<%=h c.employee.last_name
%></td><td><%=h c.period %></td>
    </tr>
  <% end %>
</table>

How to include sum(period)?