I got the following error.
My guess is that there is a 'private' setting in the lineitem class, but
I can't find it 
"NoMethodError in Carts#show
Showing /Users/kwadwoadu/src/ruby-test/depot/app/views/carts/show.html.erb where line #11 raised:
private method `total_price' called for #<LineItem:0x007f98e108a0a8> Extracted source (around line #11):
8: <tr> 9: <td><%= item.quantity %>×</td> 10: <td><%= item.product.title %></td> 11: <td class="item_price"><%= number_to_currency(item.total_price) %></td> 12: </tr> 13: <% end %> 14: <tr class="total_line">"
