In Odoo 8 there is an option to create invoices from Delivery orders, and this makes the 'source' of the invoice the delivery/picking number e.g. WH/OUT/0001. 

We are now looking to move to Odoo 9. Here it seems to only generate invoices from sales orders SO, and so on the invoice it has the 'source' as the SO no. for example SO0001

I would like to add the delivery/picking number reference to the invoice, as well as the SO number. It is the field 'name' from the Model 'stock.picking', but is how do I call this from the invoice?

 Thanks in advance for your help.