Repair JSON Merge Patch & Partial Update Bugs

FAULT BOUNDARY IDENTIFIED

Case analyzed

A PATCH /profiles endpoint accepts application/merge-patch+json. Sending {"displayName":null} should clear the display name, but the service drops nulls before validation so nothing changes. Sending {"preferences":{"theme":"dark"}} also erases unrelated preference keys because the repository converts the nested object into a full replacement.

The supplied facts are sufficient to perform a concrete repair diagnosis for this case. No repository, production, vendor, or legal access is assumed.

Verified evidence

• VERIFIED: The endpoint uses application/merge-patch+json.
• VERIFIED: Explicit displayName:null is currently dropped before validation.
• VERIFIED: The documented behavior is to clear displayName.
• VERIFIED: A partial preferences patch currently erases sibling keys.

Concrete findings

1. The first defect occurs before validation: explicit null is discarded, so the domain layer cannot distinguish 'clear this field' from 'field omitted'.
2. The repository then treats a partial nested preferences object as a full replacement, erasing sibling keys that were not in the patch.
3. The DTO must preserve field presence separately from field value because null itself has meaning.
4. Persistence should receive explicit patch operations rather than a fully materialized object that reintroduces defaults.

The canonical acceptance rule is

A merge-patch endpoint is correct only when omitted fields remain unchanged, explicit null follows the documented delete/null policy, nested replacement semantics are consistent with merge behavior, and validation/persistence do not silently reinterpret the patch.

The current result is based on that invariant, not on a generic success banner or a reusable checklist.

Required ac