public function approve($invitationKey)
 {
     $invitation = Invitation::with('invoice.invoice_items', 'invoice.invitations')->where('invitation_key', '=', $invitationKey)->firstOrFail();
     $invoice = $invitation->invoice;
     if ($invoice->is_quote && !$invoice->quote_invoice_id) {
         Event::fire(new QuoteApproved($invoice));
         Activity::approveQuote($invitation);
         $invoice = $this->invoiceRepo->cloneInvoice($invoice, $invoice->id);
         Session::flash('message', trans('texts.converted_to_invoice'));
         foreach ($invoice->invitations as $invitationClone) {
             if ($invitation->contact_id == $invitationClone->contact_id) {
                 $invitationKey = $invitationClone->invitation_key;
             }
         }
     }
     return Redirect::to("view/{$invitationKey}");
 }