Find original sales line by return sales line

returnSalesLine.InventTransIdReturn = originalSalesLine.InventTransId

protected SalesId getOriginalSalesId(SalesId _returnSalesId)
{
    SalesId             ret;
    SalesLine           salesLine;
    InventTransOrigin   inventTransOrigin;

    select firstOnly RecId from salesLine
        where salesLine.SalesId             == _returnSalesId
            &&salesLine.InventTransIdReturn != ''
        join RecId, ReferenceId from inventTransOrigin
            where inventTransOrigin.InventTransId       == salesLine.InventTransIdReturn
                &&inventTransOrigin.ReferenceCategory   == InventTransType::Sales;

    if (inventTransOrigin)
    {
        ret = inventTransOrigin.ReferenceId;
    }

    return ret;
}