Все дело в fetchMode
Попробуйте вот так
static void Job1(Args _args)
{
Query q;
QueryBuildDataSource qbdsISum;
QueryBuildDataSource qbdsITable;
QueryBuildDataSource qbdsIDim;
;
q = new Query();
qbdsISum = q.addDataSource(tableNum(InventSum));
qbdsITable = qbdsISum.addDataSource(tableNum(InventTable));
qbdsITable.fetchMode(QueryFetchMode::One2One);
qbdsITable.relations(true);
qbdsIDim = qbdsISum.addDataSource(tableNum(InventDim));
qbdsIDim.fetchMode(QueryFetchMode::One2One);
qbdsIDim.relations(true);
box::info(q.dataSourceNo(1).toString());
}