In config/routes.rb:

  map.resources :offerings, :has_many => [:allocations]

  map.resources :allocations

In AllocationsController:
  def new
    @allocation = Allocation.new
    @offering = Offering.find_by_id(params[:offering_id])
  end

Gets this error:

 Showing app/views/allocations/_allocation_form.html.erb where line #18 raised:

undefined method `allocation_offering_path' for #<ActionView::Base:0x102ecb780>

Extracted source (around line #18):

15:     $('input[name=new_underwriter]').live('change', maintain_sub_underwriter_list);
16:   });
17: </script>
18: <% form_for([@allocation, @offering]) do |f| %>
19:   <%= f.error_messages %>
20:
21:     <div id="main_wizard_div">