Throw error when creating hasOne relation that already exists

Previously, using `create` on a hasOne relation when a related model
already exists would silently create a second record, corrupting the
relationship. Now throws a GraphQL error guiding users to use `upsert`.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: spawnia

src/Execution/Arguments/NestedOneToOne.php

@@ -2,6 +2,7 @@
 
 namespace Nuwave\Lighthouse\Execution\Arguments;
 
+use GraphQL\Error\Error;
 use Illuminate\Database\Eloquent\Relations\HasOne;
 use Illuminate\Database\Eloquent\Relations\MorphOne;
 use Nuwave\Lighthouse\Support\Contracts\ArgResolver;
@@ -22,6 +23,13 @@ public function __invoke($model, $args): void
         assert($relation instanceof HasOne || $relation instanceof MorphOne);
 
         if ($args->has('create')) {
+            if ($relation->exists()) {
+                $relatedClass = class_basename($relation->getRelated());
+                $parentClass = class_basename($model);
+
+                throw new Error("Cannot create a related model: a {$relatedClass} already exists for this {$parentClass}. Use upsert to modify the existing model.");
+            }
+
             $saveModel = new ResolveNested(new SaveModel($relation));
 
             $saveModel($relation->make(), $args->arguments['create']->value);

tests/Integration/Execution/MutationExecutor/HasOneTest.php

@@ -331,6 +331,42 @@ public function testUpdateWithNewHasOne(string $action): void
         ]);
     }
 
+    /** @dataProvider existingModelMutations */
+    #[DataProvider('existingModelMutations')]
+    public function testCreateHasOneWhenAlreadyExists(string $action): void
+    {
+        $task = factory(Task::class)->create();
+        $this->assertInstanceOf(Task::class, $task);
+
+        $task->post()
+            ->save(
+                factory(Post::class)->create(),
+            );
+
+        $this->graphQL(/** @lang GraphQL */ <<<GRAPHQL
+        mutation {
+            {$action}Task(input: {
+                id: 1
+                name: "foo"
+                post: {
+                    create: {
+                        title: "bar"
+                    }
+                }
+            }) {
+                id
+                name
+                post {
+                    id
+                    title
+                }
+            }
+        }
+        GRAPHQL)->assertGraphQLErrorMessage('Cannot create a related model: a Post already exists for this Task. Use upsert to modify the existing model.');
+
+        $this->assertSame(1, Post::count());
+    }
+
     /** @dataProvider existingModelMutations */
     #[DataProvider('existingModelMutations')]
     public function testUpdateAndUpdateHasOne(string $action): void