i want to select variant of a product from a one2many field  created in sale order  just like selecting product in sale order line ...bt getting error like this 

class variants_line(models.Model):
    _inherit = 'sale.order'

    variant_name = fields.One2many('var.var', 'related_sale_order')


class variant(models.Model):
    _name = 'var.var'
    _inherit = 'sale.order.line'

    contract_no = fields.Char(string="Contract NO",default=lambda self: _('con'))
    related_sale_order = fields.Many2one('sale.order',default=lambda self: _('soo'))

    @api.multi
    def _default_order(self):
        return self.related_sale_order

    _defaults = {

        'order_id': _default_order,
    }
Expected singleton: var.var('lang', 'search_default_my_sale_orders_filter', 'tz', 'uid', 'params', 'mail_create_nolog')