Removing unimportant constraint that causes trouble.

app/model/entity/ExerciseFileLink.php

@@ -13,8 +13,11 @@
  * I.e., when Assignment is created from Exercise, the links are copied (immediately) as well.
  * The link of an exercise may be updated, but the link of an assignment is immutable.
  * @ORM\Entity
- * @ORM\Table(uniqueConstraints={@ORM\UniqueConstraint(columns={"key", "exercise_id"}),
- *                               @ORM\UniqueConstraint(columns={"key", "assignment_id"})})
+ * @ORM\Table(uniqueConstraints={@ORM\UniqueConstraint(columns={"key", "exercise_id"})})
+ *
+ * Note: technically, there should be also unique constraint on (key, assignment_id); however, it causes more problems
+ * than it solves. During sync operation, the assignment-related links are cleared and re-filled. Since the Doctrine
+ * performs inserts before deletes, and we do not want to flush in the middle, it causes unique constraint violation.
  */
 class ExerciseFileLink implements JsonSerializable
 {

migrations/Version20251214183529.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Migrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20251214183529 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return '';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('DROP INDEX UNIQ_1187F7758A90ABA9D19302F8 ON exercise_file_link');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('CREATE UNIQUE INDEX UNIQ_1187F7758A90ABA9D19302F8 ON exercise_file_link (`key`, assignment_id)');
+    }
+}