Hi
according to this reference: https://www.mongodb.com/blog/post/how-to-select--for-update-inside-mongodb-transactions

When I lock a document with a field with a new ObjectID, the whole document is locked!

Idea :

Operations:
i have three fields : A , B , C
I locked field A with new ObjectID in transaction T1.
i locked field B with new ObjectID in transaction T2.

Behaviors (high performance) :
- In transaction T2: if field A is updated, writeConflict error occurs.
- In transaction T1: if field B is updated, writeConflict error occurs.
- Outside of transactions: If field A is updated, it waits for T1.
- Outside of transactions: If field B is updated, it waits for T2.
- T1 and T2 transaction can be commit at same time. (high performance)

https://www.mongodb.com/community/forums/t/lock-on-sub-document-in-mongo/140245