Hi,

You can use Delegation Inheritance for this case. In delegation inheritance, you create a new object that delegates some of its functionality to an existing object.

Here's an example : 

class MaintenanceRequest(models.Model):

    _name = 'maintenance.request'

    _inherit = 'maintenance.equipment'

For more information please refer to this blog: Different Types of Inheritance in Odoo

Hope it helps

Hi Nasrat 

Try to add a serial_no field in maintenance request and use related to fetch its value based on maintenance.equipment 

Class MaintenanceRequest(models.Model):

_inherit = 'maintenance.request'

serial_no = fields.Char(string='Serial No',related='equipment_id.serial_no')

Regards