how to get the index of the element passed to the render view ?

I am displaying a collection of expenses in a form

the view in which the fom is displayed :
...
        = simple_form_for(@project, :html => {:class => 'form-stacked' })
do |f|
          - unless @project.expenses.count > 0
            No expenses
          - else
            #expenses
              = f.fields_for :expenses do |f|
                = render "backoffice/projects/expense_fields", :f => f
        .clearfix

and the expense_fields partial is :

.row
  = f.input :label, :wrapper => :nolabel, :input_html => { :class =>
"span3" }

I would like to test when the first expense item is displayed :

.row
        - "if first expense.. test
            = f.input :label, :input_html => { :class => "span3" }
# use the standard wrapper with labels
        - else
      = f.input :label, :wrapper => :nolabel, :input_html => { :class
=> "span3" } # no label wrapper

is there any way to do it ?

thanks for your feedback