New Doctype not update from another doctype

Hello. I am working on erpnext 13.18. I created a new child doctype where the key is a link to Sales Invoice. I assumed that when I created a new sales invoice, this field in the new doctype would automatically update, but it didn’t. How can I do it?

can you give further explanation for what are you trying to accomplice.

I created a doctype to hold the item group of a sales invoice and other related data. This doctype has fields like Sales Invoice, Item Group, Tax. After creating a new sales invoice and saving it in the table of the new doctype no information appears.

why are you creating a new doctype to save some data ? why not adding some fields to the sales invoice itself ?
otherwise you have to add a script that change data in new doctype when it does in sales invoice

because my company needs to store the invoice where the items are grouped in a single table.