How do you populate custom fields from Item to Sale Order Item on Sale Order?

[James_Houx]

I think I finally figured it out from finding this Discussion:

How to use "fetch from" function when pulling data from other DocType

In DocType “Sales Invoice Item” I added two fields, country_of_origin and customs_tariff_number. The two field are of “Link” type, with correct options inserted. What should I put in “Fetch From” in order to get the fields automatically fetched when I create a new invoice? These fields are filled in for each item. I wrote “item.customs_tariff_number” but it doesn’t work. [erpnext]

In Fetch From, you use the syntax “[source_doctype_primary_index_fieldname].[field_name]”

Personally I think this is way too “magical” looking and non-intuitive for an experienced programmer. Is this even documented anywhere? I’ve spent an immense amount of time trying to figure this out.