Correct way to do a PATCH upsert request - rest

Question

What is the proper way to request the following upsert to a REST API?
I'm struggling to structure a NoSQL collection, which is based on the most requested returns to the front-end application.
Suppose you have the following document:
{
'user' : {
'private_comments': [
//object available only to user 1
{
'id': 1,
'bar': 'He is very good',
'...': '...'
},
//object available only to user 2
{
'id': 2,
'bar': 'He is very bad',
'...': '...'
}
],
'public_comments': '' //infos available to all users
}
}
It is needed to upsert an element to the user.private_comments array.
According to https://www.rfc-editor.org/rfc/rfc6902#appendix-A.5, I could request a replace instruction PATCHing the following data:
{ 'op': 'replace', 'path': '/user/comments/$index', 'value': 'Actually, he is OK'}
The problem is that '$index' is unknown in this case.
A possible solution that I came up with was to create something like the following operation:
{ 'op': 'upsert', 'find_by': 'id', 'find': 1, 'path': '/user/comments', 'value': 'Nope, he really sucks' }
However, the user implementing the API should't provide the id value inside the PATCH request, because this value is already accessible via the receive token. Should I simplify the operation to:
{ 'op': 'upsert', 'path': '/user/comments', 'value': 'Nope, he really sucks' }
and treat it at the backend, so when it's and upsert operation without 'find' and 'find_by' variables I assume 'find_by': 'id' and 'find': value_from_token?
Also, I cannot do a simple GET/UPDATE at the hole document because the user doesn't receive the hole document, so an update would compromise the data.