As described here, I have created a many2one field with custom selection function:

def _buyer_selection(self, cr, uid, context=None):
    partner_obj = self.pool.get('res.partner');
    res = [];
    partner_ids = partner_obj.search(cr, uid, [('buyer','=',True)]);
    for partner in partner_obj.browse(cr, uid, partner_ids, context=context):
        res.append((partner.id, partner.buyer_code))
    return res;

_columns = {
    'buyer_id': fields.many2one('res.partner', 'Buyer', selection=_buyer_selection),
}

But in the view

    <field name="buyer_id"/>

It still acts like a normal many2one field. All partners selectable, no buyer_code displayed.
Although:

The debug view shows a perfectly working selection...

What am I missing?!

EDIT:

 <field name="buyer_id" widget="selection"/>

Selection widget doesn't work.
The field will become a selection field indeed, but the custom selection function seems to be overridden: