docs: add M2M tags refactor task proposal to TODO

TODO.md

@@ -33,6 +33,84 @@
 - [x] Guard `deploy-db` against overwriting existing remote database
 - [x] Update README.md and docs/SERVER_SETUP.md to reflect current structure
 
+## Refactor: M2M tags via `tags` + `thesis_tags` junction table
+
+The current schema stores keywords in a `keywords` table joined via `thesis_keywords`.
+The field column is named `keyword` (not `name`), breaking the naming convention used by
+every other lookup table (`orientations.name`, `format_types.name`, etc.).
+More critically, `buildSearchConditions` and the view `v_theses_full` filter keywords
+through `GROUP_CONCAT` strings with `LIKE`, bypassing the junction table entirely.
+
+Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis_tags`,
+`tags.name`), add the missing index, and rewrite all tag queries to use a proper JOIN.
+
+### 1 — Schema migration (`storage/schema.sql` + live DB)
+
+- [ ] Rename table `keywords` → `tags`; rename column `keywords.keyword` → `tags.name`
+- [ ] Rename junction table `thesis_keywords` → `thesis_tags`; rename FK column
+      `thesis_keywords.keyword_id` → `thesis_tags.tag_id`
+- [ ] Composite PK on `thesis_tags(tag_id, thesis_id)` (tag first — matches the lookup
+      pattern `WHERE t.name = ?`)
+- [ ] Add index `idx_tags_name ON tags(name)` (supports exact-match lookup on insert/find)
+- [ ] Update `idx_thesis_keywords_*` index names → `idx_thesis_tags_thesis`,
+      `idx_thesis_tags_tag`
+- [ ] Update view `v_theses_full` / `v_theses_public`: replace
+      `LEFT JOIN keywords k ON tk.keyword_id = k.id … GROUP_CONCAT(DISTINCT k.keyword)`
+      with `LEFT JOIN tags t ON tt.tag_id = t.id … GROUP_CONCAT(DISTINCT t.name)`
+- [ ] Write and test a SQLite migration script
+      (`storage/migrations/001_rename_keywords_to_tags.sql`)
+
+### 2 — `src/Database.php`
+
+- [ ] `findOrCreateKeyword()` → `findOrCreateTag()`: query `tags` table, column `name`
+- [ ] `getUsedKeywords()` → `getUsedTags()`: rewrite to use proper M2M JOIN instead of
+      querying the view:
+      ```sql
+      SELECT DISTINCT t.* FROM tags t
+      JOIN thesis_tags tt ON t.id = tt.tag_id
+      JOIN theses th ON tt.thesis_id = th.id
+      WHERE th.is_published = 1 ORDER BY t.name
+      ```
+- [ ] `buildSearchConditions`: replace the `keywords LIKE :keyword` view-string hack with
+      a subquery using the junction table:
+      ```sql
+      EXISTS (
+        SELECT 1 FROM thesis_tags tt
+        JOIN tags t ON t.id = tt.tag_id
+        WHERE tt.thesis_id = theses.id AND t.name LIKE :keyword ESCAPE '\'
+      )
+      ```
+      (search still runs on `v_theses_public`; the subquery references the base table)
+- [ ] `validateSearchParams`: rename key `'keyword'` → `'tag'` (or keep alias for
+      backwards-compat during transition)
+- [ ] Add backwards-compat alias `findOrCreateKeyword` → `findOrCreateTag` and
+      `getUsedKeywords` → `getUsedTags` (remove after all callers updated)
+
+### 3 — Admin write paths
+
+- [ ] `public/admin/actions/formulaire.php`: replace `findOrCreateKeyword` +
+      `INSERT INTO thesis_keywords` with `findOrCreateTag` + `INSERT INTO thesis_tags`
+- [ ] `public/admin/edit.php`: same replacement in keyword update block
+      (`DELETE FROM thesis_keywords` → `DELETE FROM thesis_tags`, insert loop)
+
+### 4 — Public read paths
+
+- [ ] `public/search.php`: rename `$keywords` → `$tags`; update `getUsedKeywords()` call
+      → `getUsedTags()`; rename GET param `keyword` → `tag` (keep old param as alias)
+- [ ] `public/tfe.php`: `$data['keywords']` → `$data['tags']` (view column rename)
+- [ ] `templates/search-bar.php` (if applicable): update any hardcoded `keyword` param refs
+
+### 5 — Tests
+
+- [ ] `tests/Unit/DatabaseTest.php`: add test for `findOrCreateTag` round-trip
+- [ ] `tests/Integration/SearchTest.php`: add test for tag-filter search using the new
+      subquery path (not the LIKE-on-GROUP_CONCAT path)
+
+### 6 — Fixtures / seed data
+
+- [ ] `storage/fixtures/CreateTestDatabase.php`: update to use `tags` / `thesis_tags`
+      table names and `findOrCreateTag()`
+
 ## Pending
 
 - [x] Add flake.nix for Nix-based PHP dev environment