Hi,

If you can achieve the things using the related field concept, then you can use it and if not possible you can use onchange.

Here you should understand the working of the related field, by default the related fields will be readonly field and if the end user need to edit the field, the readonly attribute has to set to False.

But once the readonly field is made editable and if the end user change the value, it will update the master record also.

Suppose if you add related field in sale.order model for a new field phone, lets say

phone = fields.Chat(related='partner_id.phone', readonly=False)

once user change value in this field, it will also update the phone field in the partner master also. But if you you use onchange here, it will not update the master.

So finally its the choice of the developer to understand the working of onchange and related field and to use according to the situation.

Thanks