Intento usar este código pero no funciona

var idUser = event.data.ref.parent.ref.parent.child('idUser').previous.val();

entonces recibo un error cuando intento usar

event.data.ref.parent.ref.parent.child('idUser').onDelete(event => {
    event.data.previous.val();
});

Este es mi código completo

exports.makePurchaseSummaryDelete = functions.database.ref('/invoice_data/{pushIdInvoice}/item/{pushIdItem}')
    .onDelete(event => {
    var name = event.data.child('itemName').previous.val();
    var quantity = event.data.child('quantity').previous.val();
    var idUser = event.data.ref.parent.ref.parent.child('idUser').previous.val();
    var refUser = db.ref('/user_data/' + idUser + '/purchaseSummary/' + name.toUpperCase());
    refUser.once("value", function(snapshotUser) {

    }, function (errorObject) {
      console.log("The read failed: " + errorObject.code);
    });

    return true;
});