link_to_add_fields "Add Field", f, :fields

application.js.coffee

$(document).on 'click', 'form .remove_fields', (event) ->
  $(this).prev('input[type=hidden]').val('1')
  $(this).closest('fieldset').hide()
  event.preventDefault()

$(document).on 'click', 'form .add_fields', (event) ->
  time = new Date().getTime()
  regexp = new RegExp($(this).data('id'), 'g')
  $(this).before($(this).data('fields').replace(regexp, time))
  event.preventDefault()

application_helper.rb

module ApplicationHelper
	def link_to_add_fields(name, f, association)
		new_object = f.object.send(association).klass.new
		id = new_object.object_id
		fields = f.fields_for(association, new_object, child_index: id) do |builder|
			render(association.to_s.singularize + "_fields", f: builder)
		end
		link_to(name, '#', class: "add_fields", data: {id: id, fields: fields.gsub("\n", "")})	
	end
end