Problem finishing cart

I am setting up a shop, it consists of products, users, a cart, using several examples and getting in hand I make some modifications, but I find myself stuck in the following: At the end of the cart I want a button that closes it and allows the user to generate another separate cart, but I can not do something that works, any suggestions / help? I pass the code to you:

views / carts / show.html.erb:

<div class="cart_item">
    <%= render "partials/cart_item" %>
</div>

views / partials / _cart_item.html.erb:

<% if !@order_item.nil? && @order_item.errors.any? %>
    <ul>
        <% @order_item.errors.full_messages.each do |msg| %>
            <li><%= msg %></li>
        <% end %>
    </ul>
<br/>
<h3>Carrito de compras</h3>
<br/>
<div>
    <%= link_to "Atras", root_path, class: 'btn btn-success' %>
</div>
<% if @order_items.size>0 %>
    <% @order_items.each do |item| %>
    <div>
        <div class="well">
            <div class="row">
                <div class="col-sm-8">
                    <h3><%= item.product.title %></h3>
                </div>
                <div class="col-sm-4">
                    <div class="pull-right">
                        <%= form_for(item, remote: true) do |f| %>
                        <%= f.hidden_field :product_id, value: item.product.id %>
                        <h5><%= item.product.price %></h5>
                        <div class="form-group">
                            <%= f.number_field :quantity, value: item.quantity.to_i, min: 1, class: 'form-control' %>
                        </div>
                        <div class="btn-group">
                            <%= f.submit "Modificar", class: 'btn btn-primary' %>
                            <%= link_to  "Eliminar", item, method: :delete, remote: true, class: 'btn btn-danger'  %>
                        </div>
                        <span>Total Price: <%= item.total_price %></span>
                        <% end %>
                    </div>
                </div>
            </div>
        </div>
        </div>
    <% end %>
    <div>
      <li><%= link_to "Finalizar compra", class: "fa fa-sign-out" %></li>
    </div>
<% else %>
<% end %>

models / order.rb

class Order < ApplicationRecord
  has_many :order_items
  before_save :set_data


  def subtotal
    order_items.collect {|order_item| order_item.valid? ? (order_item.unit_price*order_item.quantity) : 0}.sum
  end
  def total
    order_items.collect {|order_item| order_item.valid? ? (subtotal) : 0}.sum
  end
  # def usuario
  #   current_user.id
  # end
  private
    def set_data
      self[:subtotal] = subtotal
      self[:total] = total
      self[:status] = "to_checkout"
      # self[:user_id] = usuario
    end
end

I leave you the rar here with the project if you need more information: