$('tr[id=quantity-<%= @parent_quantity.id %>]').nextUntil('tr.quantity').remove();
var columns = $('table > thead > tr > th').length;
$('tr[id=quantity-<%= @parent_quantity.id %>]').nextUntil('tr.quantity').addBack().after(
  '<tr><td class="form" colspan="'+columns+'">' +
  '<div id="new-child-quantity"><%= j render partial: "quantities/new_child_form" %></div>' +
  '</td></tr>'
);