Hi,

In your code, you can see handling.group has relation with product.product not with product.template and 

It is not compulsory to add many2one in both models when you want to establish a relationship with 2 models.

You can use only one many2one field to use that model data in another model.

In a simple way,

To use Many2one you only need to define comodel name parameter in a field:

customer_id = fields.Many2one('res.partner')

To use One2many you need to define comodel name with inverse field name:

for ex:- To add custom model One2many record in res.partner:

Class Partner(models.Model):

    _inherit = 'res.partner'

    my_model_ids = fields.One2many('my.model', 'partner_id')

Class Mymode(models.Model):

        _name = 'my.model'

        partner_id = fields.Many2one('res.partner')

For many2many :

Class Mymode(models.Model):

        _name = 'my.model'

        partner_ids = fields.Many2many('res.partner')     

You can add optional name of the column referring to "these" in the table ``relation

To get more details about using these fields you can refer the comment details available in the file : 

odoo > fields.py

Class Many2one

Class One2many

Class Many2many

To get examples you can see documentation of Odoo:

https://www.odoo.com/documentation/12.0/howtos/backend.html#relational-fields