class SaleLine(models.Model):
    _inherit = "sale.order.line"
 width_id = fields.Many2one('sale.order.line.width')
 lng = fields.Float(default=1)
 formula = fields.Char(compute='_get_formula', )
 unit_id = fields.Many2one('product.uom')
 quantity_extra = fields.Float(default=1)
 quantity_sum = fields.Float(compute='_compute_quantity', store=True)

Hi,

You can make that custom field related to that bom_id field, than it will automatically get id from relation.

Field Definition: custom_id = fields.Char(related="bom_id.custom_id",store=True)

custom field should be there in object where you have to put relation.

Thanks,