diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index bd3f8256065..7d3af70382a 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -613,9 +613,13 @@ class purchase_order_line(osv.osv):
         return res
 
     def _get_uom_id(self, cr, uid, *args):
-        cr.execute('select id from product_uom order by id limit 1')
-        res = cr.fetchone()
-        return res and res[0] or False
+        model_data = self.pool.get('ir.model.data')
+        xml_id = 'product_uom_unit'
+        res = model_data.search(cr,uid, [('name', '=', xml_id)])
+        if len(res):
+            res = model_data.browse(cr, uid, res[0])
+            return res.res_id 
+        return False
     
     _columns = {
         'name': fields.char('Description', size=256, required=True),
diff --git a/addons/sale/sale.py b/addons/sale/sale.py
index 231fa2cdbf1..a150ceb3469 100644
--- a/addons/sale/sale.py
+++ b/addons/sale/sale.py
@@ -839,9 +839,13 @@ class sale_order_line(osv.osv):
         return res
 
     def _get_uom_id(self, cr, uid, *args):
-        cr.execute('select id from product_uom order by id limit 1')
-        res = cr.fetchone()
-        return res and res[0] or False
+        model_data = self.pool.get('ir.model.data')
+        xml_id = 'product_uom_unit'
+        res = model_data.search(cr, uid, [('name', '=', xml_id)])
+        if len(res):
+            res = model_data.browse(cr,uid,res[0])
+            return res.res_id
+        return False
     
     _name = 'sale.order.line'
     _description = 'Sales Order Line'