Form validate unique if change field in model

Here is my EditForm init code (Edit form of 1 category):

public function initialize(ArticleCategory $category) {
        // Slug
        $slug = (new Text('slug', [
            'class' => 'form-control',
            'placeholder' => 'Slug'
        ]))->addValidator(new PresenceOf([
                'message' => 'Slug is required'
        ]));
        if($category->hasChanged('slug')){
            $slug->addValidator(new Uniqueness([
                'message' => 'Slug exists',
                'field' => 'slug',
                'model' => 'App\Models\Entities\ArticleCategory'
            ]));
        }
        $slug->addFilter('trim');
        $this->add($slug);
        // other element
}

Controller code:

public function editAction($id) {
        // Get article category by id
        $articleCategory = $this->_articleCategoryService->getById($id);
        if (!$articleCategory) {
            return $this->show404();
        }
        $editForm = new EditForm($articleCategory);
        // Post
        if ($this->request->isPost()) {
            $postData = $this->request->getPost();
            if (!$editForm->isValid($postData)) {
                $editForm->setDefaultsFromObject($articleCategory);
            } else {
                // Form valid
                $editForm->bind($postData, $articleCategory);
                if ($articleCategory->save()) {
                    $this->flash->success("Save success.");
                    return $this->redirectToIndex();
                } else {
                    $this->flash->error("Save fail");
                }
            }
        }
        $this->view->editForm = $editForm;
}