fix: use instanceof for relation detection

src/Filters/FiltersBelongsTo.php

@@ -56,7 +56,7 @@ protected function getRelatedModel(Model $modelQuery, string $relationName, stri
     protected function getRelatedModelFromRelation(Model $model, string  $relationName): ?Model
     {
         $relationObject = $model->$relationName();
-        if (! is_subclass_of($relationObject, Relation::class)) {
+        if (! $relationObject instanceof Relation) {
             throw RelationNotFoundException::make($model, $relationName);
         }
 

tests/FilterTest.php

@@ -1,14 +1,14 @@
 <?php
 
 use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\RelationNotFoundException;
 use Illuminate\Http\Request;
 use Illuminate\Support\Carbon;
 use Illuminate\Support\Facades\DB;
 use Pest\Expectation;
 
-use function PHPUnit\Framework\assertObjectHasProperty;
-
 use Spatie\QueryBuilder\AllowedFilter;
+
 use Spatie\QueryBuilder\Enums\FilterOperator;
 use Spatie\QueryBuilder\Exceptions\InvalidFilterQuery;
 use Spatie\QueryBuilder\Filters\Filter as CustomFilter;
@@ -19,6 +19,8 @@
 use Spatie\QueryBuilder\Tests\TestClasses\Models\RelatedModel;
 use Spatie\QueryBuilder\Tests\TestClasses\Models\TestModel;
 
+use function PHPUnit\Framework\assertObjectHasProperty;
+
 beforeEach(function () {
     $this->models = TestModel::factory()->count(5)->create();
 });
@@ -908,3 +910,24 @@ public function __invoke(Builder $query, $value, string $property): Builder
 
     expect($results)->toHaveCount(2);
 });
+
+it('throws RelationNotFoundException when the relation method exists but does not return an Eloquent Relation', function () {
+    $ModelWithNonRelationMethod = new class () extends TestModel {
+        protected $table = 'test_models';
+
+        public function nonRelationMethod()
+        {
+            return new stdClass();
+        }
+    };
+
+    $ModelWithNonRelationMethod::create(['name' => 'test_user']);
+
+    $request = new Request([
+        'filter' => ['author' => '1'],
+    ]);
+
+    QueryBuilder::for($ModelWithNonRelationMethod::query(), $request)
+        ->allowedFilters(AllowedFilter::belongsTo('author', 'nonRelationMethod'))
+        ->get();
+})->throws(RelationNotFoundException::class);