fix: reorder subject index migration to be MySQL FK-safe (production hotfix)

[timh8127]

Production hotfix

db.Database.Migrate() crashes at startup with:

MySqlException: Cannot drop index 'IX_Subjects_UserId': needed in a foreign key constraint

Root cause

The AddSubjectNameUniqueIndex migration dropped IX_Subjects_UserId before creating its replacement. On MySQL that index is the sole backing for FK_Subjects_Users_UserId, so the DROP INDEX is rejected and the whole Migrate() call throws on boot.

CI didn't catch it because the test suite uses the in-memory provider, which ignores relational migrations.

Fix

Reorder Up() so the FK always has a backing index at every step:

1. AlterColumn — set the Name collation (utf8mb4_general_ci)
2. CreateIndex — composite unique IX_Subjects_UserId_Name (leading UserId can serve the FK)
3. DropIndex — drop the old single-column IX_Subjects_UserId

Down() reverses in the mirror-safe order (recreate single-column index → drop composite → revert collation).

Why this is safe to deploy

The original migration failed on its first statement (the drop), so nothing was applied and it was never recorded in __EFMigrationsHistory. The corrected migration runs from scratch on the next deploy and applies cleanly. No schema-shape change versus the original intent — only statement order differs.

Testing

• dotnet build -c Release — green, 0 warnings.
• Statement order verified by hand against MySQL's FK-index-drop rule.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Database schema optimization for improved subject data uniqueness management.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 4f5ca544-0906-41e3-a9cd-b80472ec7159

📥 Commits

Reviewing files that changed from the base of the PR and between e3e6a7b and af8eab2.

📒 Files selected for processing (1)

• WSIST/WSIST.Engine/Migrations/20260616205925_AddSubjectNameUniqueIndex.cs

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📝 Walkthrough

Walkthrough

A single EF Core migration file is updated to fix the operation sequence for MySQL compatibility. In Up, index drop, charset alteration, and composite unique index creation are reordered with inline comments. In Down, the single-column index is recreated before the composite index is dropped, and a duplicate recreation step is removed.

Changes

Migration Operation Reorder

Layer / File(s)	Summary
Up/Down operation reorder and duplicate removal WSIST/WSIST.Engine/Migrations/20260616205925_AddSubjectNameUniqueIndex.cs	Adds MySQL ordering comments to Up, compacts the IX_Subjects_UserId drop, reorders Down to recreate IX_Subjects_UserId before dropping IX_Subjects_UserId_Name, and removes the duplicate Down index recreation step.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐇 Hop, hop, the index falls first,
Then charset is tweaked — no need to thirst!
The composite blooms in its rightful place,
No duplicate steps to muddy the race.
Down reverses in orderly grace. ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: reordering a database migration to fix a MySQL foreign key constraint issue that was causing production outages.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/subject-index-migration-order

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