How set a node as select by default in a TreeGrid ?

I’m using smartgwt 2.2,
I’ve a TreeGrid object which uses a serverside datasourde to load data:

final TreeGrid treeGrid = new TreeGrid();
treeGrid.setSelectionAppearance(SelectionAppearance.CHECKBOX); 
treeGrid.setAutoFetchData(true);
treeGrid.setLoadDataOnDemand(false);
treeGrid.setDataSource(new TreeGridDataSource());

TreeGridDataSource extends GwtRpcDataSource & loads all tree nodes data at first fetch operation .
My TreeGrid involves a checkbox for each row, & in my datasource I’ve a boolean field which I want to use it to check some nodes automatically when TreeGrid loads .
Any Idea ?

setSelectionProperty

treeGrid.setSelectionProperty("myBooleanField");

I was using a String field and found that supplying the field in the JSON data makes it true and not supplying it makes it false.