I have a custom field in res.partner to indicate whether they like to get their invoices via E-Mail or via post. Now I want this information to show in the TreeView of the invoices as a new column. 

<tree colors="blue:state == 'draft';black:state in ('proforma','proforma2','open');gray:state == 'cancel'" string="Invoice">
 <field name="partner_id" groups="base.group_user"/>
                    [...]                    
                    <field name="amount_total" sum="Total Amount"/> 
                    <field name="partner_id.x_invoice_by"/>
                    <field name="state"/>
                </tree>

View account.invoice.tree

This results in an error (Field "partner_id.x_invoice_by" does not exist). I tried to include the information with a reference field with this selection: [('res.partner', 'partner_id.x_invoice_by')] , but had no luck either, the column appears, but is empty for all invoices.

EDIT: I'm using Odoo 8.0 if that matters