Rails Invoicing App - ruby-on-rails

Question

I want to create an invoice in rails. Invoice can have items and each item will have quantity, tax & price. It's a typical invoice we see everyday.
In order to create an invoice what is the best approach.
What is the common model for invoice and items?
I know Items will be a separate model. But how can we have one view for invoice, which creates both the invoice and items added to it?
What I mean is, Inside a new invoice page, there will be list of the clients, and list of the items , But here i'm not sure how to make the association when i create invoice. Is there any good example that i can follow ?
Please I'd appreciate some Help. Or even just a walk through of the steps i need to follow in order to accomplish that...
Here's my basic ERD

Answer 1

Quite a broad question, here's what I'd do:
#app/models/invoice.rb
class Invoice < ActiveRecord::Base
belongs_to :user
has_many :line_items
has_many :items, through: :line_items
accepts_nested_attributes_for :line_items
end
#app/models/line_item.rb
class LineItem < ActiveRecord::Base
belongs_to :invoice
belongs_to :item
end
#app/models/item.rb
class Item < ActiveRecord::Base
belongs_to :company
has_many :line_items
has_many :invoices, through: :line_items
end
--
#app/models/user.rb
class User < ActiveRecord::Base
has_many :invoices
end
This will be the base level "invoice" association structure - your clients/users can be built on top of it.
Your routes etc can be as follows:
#config/routes.rb
resources :invoices
#app/controllers/invoices_controller.rb
class InvoicesController < ApplicationController
def new
#invoice = current_user.invoices.new
#invoice.line_items.build
end
def create
#invoice = current_user.invoices.new invoice_params
#invoice.save
end
end
Then your view will be something like this:
#app/views/invoices/new.html.erb
<%= form_for #invoice do |f| %>
<%= f.fields_for :line_items do |l| %>
<%= f.text_field :quantity %>
<%= f.collection_select :product_id, Product.all, :id, :name %>
<% end %>
<%= f.submit %>
<% end %>
This would create the corresponding #invoice, with which you'll be able to call as follows:
#user.invoices.first
Apart from this, I don't have anywhere enough specific information to help specifically

Answer 2

May I recommend using the payday gem? I have created invoice models in the past applications and I'll tell you what, it can get pretty tricky sometimes depending on the type of application you're building. But the reason I like using this gem besides the convenience factor is that it can also render your invoices as a customizable PDF.
It makes adding items to the invoice a breeze as well, for example from their GitHub page:
invoice = Payday::Invoice.new(:invoice_number => 12)
invoice.line_items << Payday::LineItem.new(:price => 20, :quantity => 5, :description => "Pants")
invoice.line_items << Payday::LineItem.new(:price => 10, :quantity => 3, :description => "Shirts")
invoice.line_items << Payday::LineItem.new(:price => 5, :quantity => 200, :description => "Hats")
invoice.render_pdf_to_file("/path/to_file.pdf")