give the _rec_name for your many2one model. here is an example done for pos product lines. GIve field name needed to display on rec_name

class PosOrderLine(models.Model):
    _name = "pos.order.line"
    _description = "Lines of Point of Sale"
    _rec_name = "product_id"

or you should override the name_get method for your custom model. for eg:

@api.multi
def name_get(self):
    if not self._context.get('employee_id'):
        # leave counts is based on employee_id, would be inaccurate if not based on correct employee
        return super(HolidaysType, self).name_get()
    res = []
    for record in self:
        name = record.name
        if not record.limit:
            name = "%(name)s (%(count)s)" % {
                'name': name,
                'count': _('%g remaining out of %g') % (record.virtual_remaining_leaves or 0.0, record.max_leaves or 0.0)
            }
        res.append((record.id, name))
    return res

This is the display set for showing remaining leaves in Leave Request's leave type selection.