Insert/update child tables

if(purchTable.purchTable_ExtFields().RecId != 0)
{
PurchTable_Ext extFields = purchTable.purchTable_ExtFields();
extFields.selectForUpdate(true);
extFields.OfferNumber = agreementHeaderDefault.ExternalReference;
extFields.doUpdate();
}
else
{
PurchTable_Ext extNew;
extNew.OfferNumber = agreementHeaderDefault.ExternalReference;;
purchTable.packAGSPurchTable_ExtFields(extNew);
extNew.SysExtensionSerializerExtensionMap::insertAfterBaseTable(purchTable);
}