How to update a field on invoice based on a field on sale order

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

Value :

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 :