Fixed: add an exclude constraint to ensure on db side that there is only one step waiting for a transition

src/Bundle/ChillMainBundle/migrations/Version20220711150006.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Chill\Migrations\Main;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20220711150006 extends AbstractMigration
+{
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE chill_main_workflow_entity_step
+            DROP CONSTRAINT chill_custom_only_one_step_opened');
+    }
+
+    public function getDescription(): string
+    {
+        return 'Add a constraint to ensure that only one step is available at a time';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE chill_main_workflow_entity_step
+            ADD CONSTRAINT chill_custom_only_one_step_opened
+                EXCLUDE (
+                    entityworkflow_id WITH =
+                ) WHERE (transitionafter IS NULL)
+                DEFERRABLE INITIALLY DEFERRED');
+    }
+}