fix(db): restore a foreign key its own rows violate

src/Shared/Infrastructure/Database/Migrations.php

@@ -232,6 +232,44 @@ public static function getMissingForeignKeys(): array
      * @return int How many were created
      */
     private static function addMissingForeignKeys(array $keys): int
+    {
+        // Rows a missing constraint would have prevented are already in the
+        // database — that is the whole premise of repairing an install that
+        // lost its keys. InnoDB validates existing rows when a constraint is
+        // added, so with checks on it refuses the very keys most worth putting
+        // back, and the failure is only logged: the constraint is quietly lost.
+        //
+        // update() and Restore both happen to set this already, so the repair
+        // worked from there and nowhere else. Establish it here instead of
+        // depending on the caller, since the caller cannot know it is needed.
+        $checksWereOn = ((int) Connection::preparedFetchValue(
+            'SELECT @@FOREIGN_KEY_CHECKS AS value'
+        )) === 1;
+        if ($checksWereOn) {
+            Connection::execute('SET FOREIGN_KEY_CHECKS = 0');
+        }
+
+        try {
+            return self::addForeignKeysUnchecked($keys);
+        } finally {
+            if ($checksWereOn) {
+                Connection::execute('SET FOREIGN_KEY_CHECKS = 1');
+            }
+        }
+    }
+
+    /**
+     * Add the absent constraints, assuming FK checks are already off.
+     *
+     * @param array<array{
+     *     name: string, table: string, columns: array<string>,
+     *     refTable: string, refColumns: array<string>,
+     *     onUpdate: string, onDelete: string
+     * }> $keys Foreign keys to add if missing
+     *
+     * @return int How many were added
+     */
+    private static function addForeignKeysUnchecked(array $keys): int
     {
         $existing = [];
         foreach (self::captureForeignKeys() as $key) {

tests/backend/Core/Database/MigrationsTest.php

@@ -660,6 +660,59 @@ public function testRecordMigrationPromotesFailureToApplied(): void
         Connection::preparedExecute("DELETE FROM _migrations WHERE filename = ?", [$testFilename]);
     }
 
+    public function testAConstraintIsRestoredEvenWhenItsRowsViolateIt(): void
+    {
+        if (!self::$dbConnected) {
+            $this->markTestSkipped('Database connection required');
+        }
+
+        // The premise of repairing an install that lost its keys is that rows
+        // the constraint would have prevented are already there. InnoDB
+        // validates existing rows when a constraint is added, so with FK checks
+        // on it refuses exactly the keys most worth restoring (errno 1452) and
+        // addMissingForeignKeys() only logs it — the constraint is lost.
+        $before = Migrations::captureForeignKeys();
+        $target = null;
+        foreach ($before as $key) {
+            if ($key['table'] === 'texttags' && count($key['columns']) === 1) {
+                $target = $key;
+                break;
+            }
+        }
+        if ($target === null) {
+            $this->markTestSkipped('No single-column texttags constraint to exercise');
+        }
+
+        Connection::execute('SET FOREIGN_KEY_CHECKS = 0');
+        Connection::preparedExecute(
+            'INSERT IGNORE INTO texttags (TtTxID, TtT2ID) VALUES (?, ?)',
+            [999123, 999123]
+        );
+        Connection::execute('SET FOREIGN_KEY_CHECKS = 1');
+
+        try {
+            Migrations::dropAllForeignKeys();
+            Migrations::restoreForeignKeys($before);
+
+            $names = array_map(
+                static fn(array $k): string => $k['table'] . '.' . $k['name'],
+                Migrations::captureForeignKeys()
+            );
+            $this->assertContains(
+                $target['table'] . '.' . $target['name'],
+                $names,
+                'A constraint its own rows violate must still be restored'
+            );
+        } finally {
+            Connection::execute('SET FOREIGN_KEY_CHECKS = 0');
+            Connection::preparedExecute(
+                'DELETE FROM texttags WHERE TtTxID = ? AND TtT2ID = ?',
+                [999123, 999123]
+            );
+            Connection::execute('SET FOREIGN_KEY_CHECKS = 1');
+        }
+    }
+
     public function testForeignKeysSurviveDropAndRestore(): void
     {
         if (!self::$dbConnected) {