How to create an auto ID?

Dear Sabrina,
In the default values section, you can define default values with conditions; you can define 2 items with this parameters:

1. Constant: 100 - Apply if: count(parent()/tree) = 1
2. Expression: math:max(parent()/tree/tree_num) + 1

The items defined in the default values section work like this: Collect iterates over the items defined there and when it finds an item that verifies the condition defined in the "Apply if" expression, it calculates the value to apply using the Constant field or the Expression field. If you don't specify any expression in the "Apply if" field, it means "always true" (this is why you can leave it empty only for the last item, otherwise the other items won't ever be evaluated).

Thanks,
Open Foris Team