thank you @Jainesh, for what you have achieve i asked for here but topic closed by me because i was unable to make a relationship with the model in popup window (form view) and nobody Answer to that topic. after reading your Answer now i thought it can be done but still as of my knowledge level i am not able to achieve what you had, if there is an example link i will be much grateful as this is very important for me. ( i am from Oracle and there is no problem creating Child of a Child in very old Oracle Forms and currently in Oracle Apex, Oracle ADF etc. you just have to have PK & FK relationship ).
thanks again.
regards

to clear any confusion because of what i wrote, here is may be, more clearer
- i failed to create relationship with popup form view where i want an editable tree view as Child of Order Line.

regards

Hello @SmithJohn45,
It we'll not possible to develop this requirement from UI side. But here is the mind-map to achieve this.

1: Create a new model. In this example we'll named is 'specification.sheet'.
2: Now we need to link this model with 'sale.order.line' and to achieve this we'll extend the 'sale.order.line' model and add Many2one field named specification_sheet_id. But we also need to link the 'sale.order.line' to 'specification.sheet'. So, we'll add new Many2one field named sale_line_id.
3: Now, add a object type button in sale order line. Once we add a button in xml file we need to write the logic for this button which is as follow.

def action_open_specification_sheet(self):
"""
Used to open specification sheet pop-up.

Note:
1: If the sale line has specification_sheet_id field populated then we'll open the same record in pop-up window by using 'res_id' in action.
2: When we're using this button for newly created sale order line, We'll not have specification_sheet_id field populated. So in this case system will create a new record of 'specification.sheet' and we need to set default value of sale_line_id field for this record with the help of 'context' in action.
"""
self.ensure_one()
return {
'name': _('Specification Sheet'),
'view_mode': 'form',
'res_model': 'specification.sheet',
'res_id': self.specification_sheet_id.id,
'view_id': False,
'type': 'ir.actions.act_window',
'context': {
'default_sale_line_id': self.id,
},
'target': 'new'
}
4: After adding a button in view, We'll define a action confirm of the specification sheet's pop-up window. In which, we need to do two things.

A: We need to set specification_sheet_id field of sale order line (We can use sale_line_id field to identify the sale line).
B: We'll prepare a string which contains all the impotent information and create a note in 'sale.order.line'. We also need to link this line with 'specification.sheet'. So, in future we can easily modify the value of this line. To achieve this we can add a new field named custom_sol_note_id in 'specification.sheet'.

Feel free to reach out to us if you have any further questions.

huge thanks @Jainesh for very detailed explaination and guidance, will check what i can achieve from it. i really impressed with your deep concern to help us.

kind regards

thanks, but what i asked is another question. is it possible, if yes, how to?