ROR -- NoMethodError in Carts#show -- Private Method error - ruby-on-rails-3

Question

I am currently following the Agile Web Development book (Rails 3) and have been stuck with a Private Method error. After hours of research I have not been able to find a solution.
The following is the code I'm working with and have discovered the string giving the problem is:
<td class="item_price"><%= number_to_currency(item.total_price) %></td>
There is a similar problem/solution that has been posted where the solution was to place the line Item class above the private, however, the line item class is not private.
Any help would be appreciated.
CODE
<% if notice %>
<p id="notice"><%= notice %></p>
<% end %>
<!-- START_HIGHLIGHT -->
<h2>Your Cart</h2>
<table>
<!-- END_HIGHLIGHT -->
<% #cart.line_items.each do |item| %>
<!-- START_HIGHLIGHT -->
<tr>
<td><%= item.quantity %>×</td>
<td><%= item.product.title %></td>
<td class="item_price"><%= number_to_currency(item.total_price) %></td>
</tr>
<!-- END_HIGHLIGHT -->
<% end %>
<!-- START_HIGHLIGHT -->
<tr class="total_line">
<td colspan="2">Total</td>
<td class="total_cell"><%= number_to_currency(#cart.total_price) %></td>
</tr>
<!-- END_HIGHLIGHT -->
<!-- START_HIGHLIGHT -->
</table>
<!-- END_HIGHLIGHT -->
<%= button_to 'Empty cart', #cart, method: :delete,
data: { confirm: 'Are you sure?' } %>
Error
private method `total_price' called for #<LineItem
line_item model
class LineItem < ActiveRecord::Base
belongs_to :product
belongs_to :cart
attr_accessible :cart_id, :product_id
end
def total_price
product.price * quantity
end
cart model
class Cart < ActiveRecord::Base
has_many :line_items, dependent: :destroy
def add_product(product_id)
current_item = line_items.find_by_product_id(product_id)
if current_item
current_item.quantity += 1
else
current_item = line_items.build(product_id: product_id)
end
current_item
end
def total_price
line_items.to_a.sum { |item| item.total_price }
end
end
line item controller
class LineItemsController < ApplicationController
# GET /line_items
# GET /line_items.json
def index
#line_items = LineItem.all
respond_to do |format|
format.html # index.html.erb
format.json { render json: #line_items }
end
end
# GET /line_items/1
# GET /line_items/1.json
def show
#line_item = LineItem.find(params[:id])
respond_to do |format|
format.html # show.html.erb
format.json { render json: #line_item }
end
end
# GET /line_items/new
# GET /line_items/new.json
def new
#line_item = LineItem.new
respond_to do |format|
format.html # new.html.erb
format.json { render json: #line_item }
end
end
# GET /line_items/1/edit
def edit
#line_item = LineItem.find(params[:id])
end
# POST /line_items
# POST /line_items.json
def create
#cart = current_cart
product = Product.find(params[:product_id])
#line_item = #cart.add_product(product.id)
#line_item.product = product
respond_to do |format|
if #line_item.save
format.html { redirect_to #line_item.cart,
notice: 'Line item was successfully created.' }
format.json { render json: #line_item,
status: :created, location: #line_item }
else
format.html { render action: "new" }
format.json { render json: #line_item.errors,
status: :unprocessable_entity }
end
end
end
# PUT /line_items/1
# PUT /line_items/1.json
def update
#line_item = LineItem.find(params[:id])
respond_to do |format|
if #line_item.update_attributes(params[:line_item])
format.html { redirect_to #line_item, notice: 'Line item was successfully updated.' }
format.json { head :no_content }
else
format.html { render action: "edit" }
format.json { render json: #line_item.errors, status: :unprocessable_entity }
end
end
end
# DELETE /line_items/1
# DELETE /line_items/1.json
def destroy
#line_item = LineItem.find(params[:id])
#line_item.destroy
respond_to do |format|
format.html { redirect_to line_items_url }
format.json { head :no_content }
end
end
end

Answer 1

In your LineItem model total_price is defined outside of the LineItem class. So it should be:
class LineItem < ActiveRecord::Base
belongs_to :product
belongs_to :cart
attr_accessible :cart_id, :product_id
def total_price
product.price * quantity
end
end

Answer 2

You are saying that #cart.total_price is giving the error but your error showing that it is looking this function in LineItem model. It probably means you must have a total_price method in LineItem model also. Do you have that? And yes as others said, it would be better to add your controller and model codes in your post.
Edit:
Based on your update in the post, yes the total_price method should be inside in your class. does that solve the problem?