On here:
you see this code:
<%= fields_for "project[invoice_attributes]", invoice, :index => nil do |form| %> <%= form.check_box :paid %> ... <% end %>
why are they using :index => nil in the example? I mean I know that what it does is ensure that the invoice objects do not get passed an index number so that you have this: project[invoice_attributes] rather than this: project[invoice_attributes][1]. But why in the above example, do they want to achieve this? It seems as though invoice is an existing object and therefore is not a new record.