---
id: enforce-parts-session-id-invariant
name: Document and enforce parts.sessionId denormalization invariant
status: completed
depends_on: []
scope: narrow
risk: high
impact: component
level: implementation
---

## Description

The invariant stated in `sessions.md:96`: "when inserting a part, always set `sessionId` to the message's `sessionId`. Never update `messages.sessionId` without updating all child parts." However, there is no DB trigger, no CHECK constraint, no application-level transaction pattern, and no immutability guarantee for `sessionId`.

The review recommends declaring `sessionId` on both `messages` and `parts` as **immutable after creation**, which eliminates the update problem. Define the application-level contract for part insertion and add an explicit "IMMUTABLE" note.

## Acceptance Criteria

- [ ] `sessions.md` documents that `sessionId` on both `messages` and `parts` is **IMMUTABLE after creation**
- [ ] The column spec for `messages.sessionId` and `parts.sessionId` includes an IMMUTABLE annotation
- [ ] The application-level contract for part insertion is documented: "read the message's `sessionId` and set it on the part within the same transaction"
- [ ] The denormalization rationale is preserved but the invariant enforcement mechanism is explicit
- [ ] Note added: "No DB trigger enforces this — immutability is respected by application layer; direct SQL must not update sessionId"

## References

- docs/reviews/storage-architecture-review-2026-04-21.md#C04
- docs/architecture/storage/sessions.md:96
- docs/architecture/storage/sessions.md:105

## Notes

> To be filled by implementation agent

## Summary

> To be filled on completion