Help for dynamic subforms in Rails

_line_item.html.erb

<!--
  In this file, the fields can be set to "required", but that will
  cause issues.  Specifically, when someone clicks "Remove" the fields
  will be hidden and unless the "required" prop is set to false the form
  will not submit.  Also note that when showing this line if the object
  is marked_for_destruction? it should be hidden. The javascript below
  will handle both these issues.
-->
<tr class="line_item subform fields" id="line_item_<%= f.object.id %>">
  <td>
    <%= f.text_field :field_1 %>
  </td>
  <td>
    <%= f.text_field :field_2 %>
  </td>
  <td>
    <%= remove_child_link 'Remove', f %>
  </td>
</tr>

_sample_form_excerpt.html.erb

  <!-- 
    Note that this belongs in a form where "f" is the form variable.
    In this case, we are dealing with a child model called LineItem.
    Make careful note of the pluralization of "line_item" below - some
    must be plural and others singular.  See the _line_item.html.erb
    file for an example of the actual subform.  Also note that you
    must allow for "id" and "_destroy" fields in your strong parameters
    for line_items_attributes.
  -->
  <fieldset>
    <legend>Line Items</legend>
    <table id='line_items_tbl'>
      <thead>
        <tr><th>Field 1</th><th>Field 2</th><th>Actions</th></tr>
      </thead>
      <tbody id="line_items">
        <%= f.fields_for :line_items do |li_form|%>
          <%= render partial: "line_item", locals: { f: li_form } %>
        <% end %>
      </tbody>
    </table>
    <%= add_child_button 'Add A Line Item', f, :line_items, partial: 'line_item' %>
  </fieldset>

application_helper.rb

module ApplicationHelper
  def remove_child_link(name, f, options = {})
    confirm = options.delete(:confirm)
    confirm = confirm.nil? ? true : confirm
    f.hidden_field(:_destroy) + link_to(name, '#', onclick: "if (#{confirm ? 'false' : 'true'} || confirm('Do you really want to remove this item?')) remove_fields(this); return false;")
  end

  def add_child_link(name, f, method, options = {})
    fields = new_child_fields(f, method, options)
    link_to(name, '#', onclick: "insert_fields(this, \"#{method}\", \"#{escape_javascript(fields)}\"); return false;")
  end

  def new_child_fields(form_builder, method, options = {})
    options[:object] ||= form_builder.object.class.reflect_on_association(method).klass.new
    options[:partial] ||= method.to_s.singularize
    options[:form_builder_local] ||= :f
    form_builder.fields_for(method, options[:object], :child_index => "new_#{method}") do |f|
      render(:partial => options[:partial], :locals => { options[:form_builder_local] => f })
    end
  end
end

standard.js

// Requires jquery

// helpers for nested forms
function insert_fields(link, method, content) {
  var new_id = new Date().getTime();
  var regexp = new RegExp("new_" + method, "g")
  if (content.match(/^\s*<tr\b/)) {
    // Add it to a table
    $(content.replace(regexp,new_id)).appendTo($(link).parents('fieldset').children('table:last'));
  } else if (content.match(/^\s*<li\b/)) {
    // Add it to a ul
    $(content.replace(regexp,new_id)).appendTo($(link).parents('fieldset').children('ul:last'));
  } else {
    $(content.replace(regexp,new_id)).insertAfter($(link).parents('fieldset').children(':last'));
  }
}

function remove_fields(link) {
  $('input[type=hidden][name*="[_destroy]"]', $(link).parent()).val('true');
  if ($(link).parents('.fields').size()>0) {
    $(link).parents('.fields').first().hide();
    $('input, select, textarea',$(link).parents('.fields').first()).prop('required',false);
  }
}

function hide_removed_records() {
  $('input[type=hidden][value=true][name$="[_destroy]"], input[type=checkbox][checked][value=1][name$="[_destroy]"]').each(function() {
    if ($(this).parents('.fields').size()>0) {
      $(this).parents('.fields').first().hide();
      $('input',$(link).parents('.fields').first()).prop('required',false);
    }
  });
}

$(document).ready(function() {
  hide_removed_records();
});

This code is used to handle add/remove for child records in a Ruby on Rails form. As an example, I'm using a child model called LineItem. Each file here is necessary. You must modify the form and subform to suit your particular data model, and include the relevant code in your ApplicationHelper as well as in your javascript directory. This code absolutely relies on jQuery.

Please see updated code:
https://gist.github.com/mdchaney/f25b2afd8f1253d07bd234a1b3f00a93