_form.html.erb

<%= simple_form_for(@invoice) do |f| %>
    <%= f.error_notification %>

    <div class="form-inputs">
        <%= f.input :address_sender %>
        <%= f.input :address_recipient %>
        <%= f.simple_fields_for :invoice_line_items do |invoice_line_items_form| %>
            <div class="duplicatable_nested_form">
                <%= invoice_line_items_form.input :description %>
                <%= invoice_line_items_form.input :price %>
                <%= invoice_line_items_form.input :amount %>
                <% if invoice_line_items_form.object.new_record? %>
                    <%= link_to "Remove", "", :remote => true, :class => "destroy_duplicate_nested_form" %>
                <% else %>
                    <%= link_to "remove", invoice_invoice_line_item_path(@invoice, invoice_line_items_form.object), :method => :delete, :remote => true, :class => "destroy_duplicate_nested_form" %>
                    <%= invoice_line_items_form.input :id, as: :hidden %>
                <% end %>
            </div>
        <% end %>
    </div>
    <div class="form-actions">
        <%= link_to "Add Another Item", "", :class => "duplicate_nested_form" %>
        <%= f.button :submit %>
    </div>
<% end %>

invoice.js.coffee

# Place all the behaviors and hooks related to the matching controller here.
# All this logic will automatically be available in application.js.
# You can use CoffeeScript in this file: http://coffeescript.org/

jQuery ($) ->
  $(document).ready ->
    if $('.duplicatable_nested_form').length

      nestedForm = $('.duplicatable_nested_form').last().clone()

      $(".destroy_duplicate_nested_form:first").remove()

      $('.destroy_duplicate_nested_form').live 'click', (e) ->
        $(this).closest('.duplicatable_nested_form').slideUp().remove()

      $('.duplicate_nested_form').click (e) ->
        e.preventDefault()

        lastNestedForm = $('.duplicatable_nested_form').last()
        newNestedForm  = $(nestedForm).clone()
        formsOnPage    = $('.duplicatable_nested_form').length

        $(newNestedForm).find('label').each ->
          oldLabel = $(this).attr 'for'
          newLabel = oldLabel.replace(new RegExp(/_[0-9]+_/), "_#{formsOnPage}_")
          $(this).attr 'for', newLabel

        $(newNestedForm).find('select, input').each ->
          oldId = $(this).attr 'id'
          newId = oldId.replace(new RegExp(/_[0-9]+_/), "_#{formsOnPage}_")
          $(this).attr 'id', newId

          oldName = $(this).attr 'name'
          newName = oldName.replace(new RegExp(/\[[0-9]+\]/), "[#{formsOnPage}]")
          $(this).attr 'name', newName

        $( newNestedForm ).insertAfter( lastNestedForm )

invoice_controller.rb

class InvoicesController < ApplicationController
  before_filter :authenticate_user!
  before_action :set_invoice, only: [:show, :edit, :update, :destroy]
  

  # GET /invoices
  # GET /invoices.json
  def index
    @invoices = current_user.invoices
  end

  # GET /invoices/1
  # GET /invoices/1.json
  def show
  end

  # GET /invoices/new
  def new
    @invoice = Invoice.new
    @invoice.invoice_line_items.build
  end

  # GET /invoices/1/edit
  def edit
  end

  # POST /invoices
  # POST /invoices.json
  def create
    @invoice = current_user.invoices.new(invoice_params)
    @invoice.status = "Pending" 
    respond_to do |format|
      if @invoice.save
        format.html { redirect_to @invoice, notice: 'Invoice was successfully created.' }
        format.json { render action: 'show', status: :created, location: @invoice }
      else
        format.html { render action: 'new' }
        format.json { render json: @invoice.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /invoices/1
  # PATCH/PUT /invoices/1.json
  def update
    respond_to do |format|
      if @invoice.update(invoice_params)
        format.html { redirect_to invoices_path, notice: 'Invoice was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @invoice.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /invoices/1
  # DELETE /invoices/1.json
  def destroy
    @invoice.destroy
    respond_to do |format|
      format.html { redirect_to invoices_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions and check if the user has access to it.
    def set_invoice
      @invoice = current_user.invoices.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def invoice_params
      params.require(:invoice).permit(:address_sender, :address_recipient, :status, :user_id, :customer_id, invoice_line_items_attributes: [:id, :description, :price, :amount])
    end
end