AX 2012 R3 Line by line invoicing the sales order using X++ code

static void RB_PostSalesInvoice(Args _args)
{
    SalesTable salesTable;
    SalesFormLetter salesFormLetter;
    Query query;
    QueryRun queryRun;
    QueryBuildDataSource qbds;

    salesTable = SalesTable::find('20001');
    query = new Query(QueryStr(SalesUpdatePackingSlip));
    qbds = query.dataSourceTable(tableNum(SalesLine));

    // Build query range to find those lines which needs to be posted.

    qbds.addRange(fieldNum(SalesLine, SalesId)).value('20001');
    qbds.addRange(fieldNum(SalesLine, SalesStatus)).value(queryValue(SalesStatus::Backorder));
    qbds.addRange(fieldNum(SalesLine, itemId)).value('V12_IF');
    queryRun = new queryRun(query);

    salesFormLetter = SalesFormLetter::construct(DocumentStatus::Invoice);
    salesFormLetter.chooseLinesQuery(queryRun);
    salesFormLetter.update(salesTable);
}