# Record Merge

## Objective

Combine two duplicate records into one without losing anything attached.

A merge flow that picks a surviving value per field, reparents all related data, redirects old references, and can be audited afterwards.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Show the two records side by side, field by field, and require an explicit survivor for every field where they differ. Do not default to 'newest wins' silently; the user is merging because they know which one is right.
2. Reparent every related object — children, comments, attachments, activity, links, memberships — onto the survivor in a single transaction. A half-finished merge is worse than two duplicates.
3. Detect relationships that would become duplicates after reparenting (the same person as a member of both records, the same tag applied twice) and collapse them rather than violating a uniqueness constraint mid-merge.
4. Redirect old references: keep the losing record's ID resolvable and point it at the survivor, so bookmarks, external links, and API consumers do not break.
5. Lock both records for the duration of the merge and write an audit entry recording who merged what, which values survived, and when. Do not build identity or account merging here — that is owned by Account Merge — and do not add pre-create duplicate warnings, which are owned by Duplicate Detection on Create.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Concurrent edits to either record during the merge must be rejected or forced to re-read, not silently discarded.
- Merging a record into itself, or merging records the actor can only partially see, must be refused up front.
- Records in different workspaces, tenants, or permission scopes must not be mergeable without an explicit rule for which scope survives.
- Timestamps need a decision: created_at should usually take the earlier of the two, not the survivor's.
- Merges are effectively irreversible once children move. Either implement an undo window or say plainly in the confirmation that it cannot be undone.
- Very large merges — thousands of children — must run in the background with progress, not block a request until it times out.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every conflicting field requires an explicit survivor choice before the merge can run.
- [ ] All related records move to the survivor within one transaction, with no orphans left behind.
- [ ] Relationships that would duplicate after reparenting are collapsed instead of erroring.
- [ ] The losing record's ID still resolves and redirects to the survivor.
- [ ] Both records are locked during the merge and concurrent edits are rejected.
- [ ] An audit entry records the actor, both records, and the surviving values.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.