InvoiceLine

    rails generate scaffold InvoiceLine description amount:integer\
    price:decimal tax_rate:decimal belongs_to:invoice
Invoice

    rails generate scaffold Invoice due_date:datetime total:decimal recipient
Setup association between invoice and invoice lines
Show details about invoices
Create invoice with lines
Install Knockout
Simple InvoiceLine viewmodel
Show the subtotal
Update subtotal on keypress
Apply view model to each invoice line
Simple InvoiceLine viewmodel
Show the subtotal
Invoice view model with total function
Apply the Invoice view model to the view