test and impl that only one root node exists per text

in practice this means only one node with a null parentNode

app/Node/UseCases/CreateNode.php

@@ -15,6 +15,9 @@ class CreateNode
         private TextRepository $textRepo,
     ) {}
 
+    /**
+     * @throws DomainException
+     */
     public function execute(CreateNodeRequest $request): Node
     {
         $textId = $request->textId;
@@ -23,6 +26,7 @@ class CreateNode
             throw new DomainException("Text with id: $textId doesnt exist");
         }
         if ($request->parentNodeId === null) {
+            $this->validateNoOtherRootNodeExists($textId);
             return $this->nodeRepo->create(new CreateNodeDto(
                 text: $text,
                 title: $request->title,
@@ -41,4 +45,21 @@ class CreateNode
             parentNode: $parentNode,
         ));
     }
+
+    /**
+     * @throws DomainException
+     */
+    private function validateNoOtherRootNodeExists(int $textId): void
+    {
+        $nodesOfText = $this->nodeRepo->findByTextId($textId);
+        $exists = array_find(
+            $nodesOfText,
+            function (Node $node) {
+                return $node->getParentNode() === null;
+            }
+        );
+        if ($exists) {
+            throw new DomainException('A root node already exists for this text');
+        }
+    }
 }

tests/Unit/Node/UseCases/CreateNodeTest.php

@@ -95,4 +95,20 @@ class CreateNodeTest extends TestCase
             parentNodeId: 0,
         ));
     }
+
+    public function test_one_root_node_per_text(): void
+    {
+        $this->expectException(DomainException::class);
+        $this->expectExceptionMessage('A root node already exists for this text');
+        $this->useCase->execute(new CreateNodeRequest(
+            textId: 0,
+            title: 'test',
+            parentNodeId: null,
+        ));
+        $this->useCase->execute(new CreateNodeRequest(
+            textId: 0,
+            title: 'test',
+            parentNodeId: null,
+        ));
+    }
 }