As mentioned by S. LE Gal the ORM link doesn't exist between the invoice and its respective sales orders. But it does exist in the database. So, you can create a custom module that adds a new many2many field to the account.invoice model that references the sale.order model via the sale_order_invoice_rel model.

class account_invoice(osv.osv):
_inherit = "account.invoice"

_columns = {
    'sale_order_ids': fields.many2many('sale.order', 'sale_order_invoice_rel'
        , 'invoice_id', 'order_id', 'Sale Orders', readonly=True, 
        help="The sale order(s) that generated this invoice. Multiple sales '\
        'orders may have been combined for the same customer to generate one invoice."),
}

Hi,

I don't think there is a link from account.invoice to sale.order, but you have the revert link : In sale.order model, you have a many2many fields named "invoice_ids" (table : sale_order_invoice_rel).

So you can request the sale_order that have linked to a defined invoice.

Regards.