<tbody>
     <%= f.simple_fields_for :saleslines do |salesline| %>
          <%= render 'salesline_fields',f: salesline %>
        <%end%>
    <td><%= link_to_add_fields "add button", f, :saleslines %></td>

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 |salesline|
  render(association.to_s.singularize + "_fields", f: salesline)
end
link_to(name, '#', class: "add_fields", data: {id: id, fields: fields.gsub("n", "</td>")})
 end
end

<tr><td>asdasd</td>
<td class="state-1">111</td>
<td class="state-2">222</td>
<td>333</td>
<td>asdasd</td></br>
<td>asdasd</td></br>
</tr>