diff --git a/base_transaction_id/invoice.py b/base_transaction_id/invoice.py
index 7460495c..c8a71699 100644
--- a/base_transaction_id/invoice.py
+++ b/base_transaction_id/invoice.py
@@ -19,32 +19,30 @@
 #
 ##############################################################################
 
-from openerp.osv.orm import Model
-from openerp.osv import fields
+from openerp import models, fields, api
 
 
-class AccountInvoice(Model):
+class AccountInvoice(models.Model):
     _inherit = 'account.invoice'
 
-    _columns = {
-        'transaction_id': fields.char(
-            'Transaction id',
-            select=1,
-            help="Transaction id from the financial institute"),
-    }
+    transaction_id = fields.Char(string='Transaction ID',
+                                 index=True,
+                                 copy=False,
+                                 help="Transaction ID from the "
+                                      "financial institute")
 
-    def copy_data(self, cr, uid, id, default=None, context=None):
-        if default is None:
-            default = {}
-        default['transaction_id'] = False
-        return super(AccountInvoice, self).\
-            copy_data(cr, uid, id, default=default, context=context)
+    @api.multi
+    def finalize_invoice_move_lines(self, move_lines):
+        """ Propagate the transaction_id from the invoice to the move lines.
 
-    def finalize_invoice_move_lines(self, cr, uid, invoice_browse, move_lines):
-        if invoice_browse.transaction_id:
-            invoice_account_id = invoice_browse.account_id.id
-            for line in move_lines:
-                # tuple (0, 0, {values})
-                if invoice_account_id == line[2]['account_id']:
-                    line[2]['transaction_ref'] = invoice_browse.transaction_id
+        The transaction id is written on the move lines only if the account is
+        the same than the invoice's one.
+        """
+        for invoice in self:
+            if invoice.transaction_id:
+                invoice_account_id = invoice.account_id.id
+                for line in move_lines:
+                    # tuple (0, 0, {values})
+                    if invoice_account_id == line[2]['account_id']:
+                        line[2]['transaction_ref'] = invoice.transaction_id
         return move_lines