Thank you so much for taking your time and answering my question.

I believe form the logic your string of code must be correct. However the field shows up as empty when running the normal process from within invoice / send and print.

So I have doubts if the field can handle joint table request.
We are still in version 15, also this could be a limitation.

Kudos and have a good weekend

Hi,

You can use this placeholder:

{{ ', '.join(map(str, object.partner_id.child_ids.filtered(lambda c: c.type == 'invoice').mapped('id'))) }}

Hope it helps