class procurement_order(osv.osv):
_inherit = 'procurement.order'
def create_procurement_purchase_order(self, cr, uid, procurement, po_vals, line_vals, context=None):
    po_vals.update({'order_line': [(0,0,line_vals)]})
    obj_po = self.pool.get('purchase.order')
    order_id = obj_po.search(cr,uid,[('partner_id','=',po_vals['partner_id']),('origin','=',po_vals['origin'])],context=context)
    if order_id:
        old_po = order_id[0]
        obj_po.write(cr, uid, [old_po], {'order_line': [(0,0,line_vals)]},context=context)
        return old_po
    else:
        return obj_po.create(cr, uid, po_vals, context=context)