I'm trying to create a custom customer statement report. I've inherited res_partner and have created a method to return an invoice contact as a res.partner object but can get it to display in the QWEb report. How do you call this correctly so I can insert a correctly formated address?

Model:

class Res_Partner(models.Model):
    _inherit = 'res.partner'

    @api.one 
    @api.returns('res.partner')       
    def _get_postal_address(self):
              
        partner_obj = self.env['res.partner']
        add_ids = self.address_get(adr_pref=['invoice']) or {}
        add_id = add_ids['invoice']
        return partner_obj.browse(add_id) 

XML:

<address t-field="_get_postal_address()" t-field-options="{&quot;widget&quot;: &quot;contact&quot;, &quot;fields&quot;: [&quot;address&quot;, &quot;name&quot;], &quot;no_marker&quot;: true}"/>
...

I've tried t-esc and t-raw. And also tried creating a new field on res_partner but I either does not fire the method or if it does if creates an error.

For testing purposes I've made sure the record has an contact of type invoice.