_form_new.html.erb

<tr>
  <td style="width: 25%" class="align-right sortable"><%= text_field_tag 'program_event[entrance_fees][][price]', nil, :class => 's' %> Kč</td>
  <td style="width: 50%" class="sortable">
    <!-- <span class="drag_handle">&nbsp;&nbsp;&nbsp;&nbsp;</span> -->
    <%= text_field_tag 'program_event[entrance_fees][][description]', nil %>
  </td>
  <td>&nbsp;</td>
</tr>

program_event.rb

class ProgramEvent < ActiveRecord::Base

  has_many :entrance_fees

  # Complex forms quite complicated, huh?
  def entrance_fees=(options)
    options.each do |new_entrance_fee|
      if entrance_fee = ProgramEntranceFee.create( new_entrance_fee )
        self.entrance_fees << entrance_fee
      end if new_entrance_fee[:price] && !new_entrance_fee[:price].blank?
    end
  end

end

the_log.log

Processing Admin::ProgramEventsController#update (for 127.0.0.1 at 2008-11-26 20:18:24) [PUT]
  Parameters: {"program_event"=>{"entrance_fees"=>[{"price"=>"7000", "description"=>"Seven thousand"}, {"price"=>"8000", "description"=>"Eight thousand"}], ...}, }