Current infra does not capture a situation when we have a state:

id: 1 unique-value: A; id: 2, unique-value: B

and following put:

id: 1 unique-value: B; id: 2, unique-value: A

We translate it as update for id1 and id2.
Which causes delete of A and create of B (if there is no special update method for the node).
Attempt to create second B fails.

Solution. Make update optional for Writers. Create special type of writers capable of updates.
For writers that can't update directly, split update request into delete and create.
Deletes are processed before updates, so the issue described will not occur anymore.