Validations of associated has_many depending on status attribute (invoices and invoice lines)

I've got the following models:

• An Invoice has several statuses: Initially it's "draft", then it can be "sent", and "payed"
• An Invoice has_many InvoiceLines. InvoiceLines have a text an开发者_开发技巧d a cost.

My question is: How can I perform the following validations?

If the Invoice Status is not draft:

• No InvoiceLines can be added to it
• The values of the invoiceLines can't be modified

Right now I've got this, but it's not working:

class Invoice < ActiveRecord::Base
  has_many :invoice_lines
  validates_associated :invoice_lines
end

class InvoiceLine < ActiveRecord::Base
  belongs_to :invoice
  validate :check_invoice_status

  def check_invoice_status
    unless self.invoice.draft?
      errors.add_to_base "Can't add or modify: the invoice status is not draft"
    end
  end
end

This does not work as intented since the validates_associated "allways fails"; once the invoice status is changed to "sent", the invoice lines are allways invalid. I only want them to be checked if "updated" or "added new".

Thaks.

use "changed?" for invoice line:

def validate_invoice_status
  if changed? and !invoice.draft?
    errors.add_to_base "Can't add or modify: the invoice status is not draft"
  end
end

try this:

    def check_invoice_status
        unless self.invoice.draft?
          self.reload if changed? && !new_record?
          errors.add_to_base("Can't add: the invoice status is not draft") if new_record?
        end
      end