Use case : I need to display an information on an invoice that is linked to the originating sale order.
How to :
Create an automated action :
Model : Journal entries = invoices
Field : Select the field on the invoice you want to update
Evaluation type : Python
Reproduce the grey part of the expression and adapt the yellow one : pick the technical name of the field on the SO you'd like to link to your new field on the invoice. If it's a many2one, add .id after the name of the field :