Ruby on Rails

[rails 3.2.2]

I perform an ajax call :
...
     $.ajax
       url: " /backoffice/expenses/rate"
       type: "GET"
       dataType: "json"
       data:
         project_id: $('#project_id').val()
         code: $(this).val()
       success: (result) ->
         alert result
         $('#expense_price').val(result)

in the controller , I am correctly receiving the js call
Started GET "/backoffice/expenses/rate?project_id=7&code=1
Processing by Backoffice::ExpensesController#rate as JSON
..
Completed 200 OK in 2004ms (Views: 0.5ms | ActiveRecord: 2.6ms)

in my controller
 def partner_rate
  ...
  @rate = "10.00"
   render :json => @rate    # for testing purpose only
 end

and the alert result is not displayed , what am I missing here ?

thanks for your feedback