Magento 2 - Assign product to category on frontend

Question

I am creating a frontend from for Sellers to add/edit their product on website, product adding/editing is working perfect. Now I have to assign that product to a category

Almost on every article and blog there is one proper solution to assign product to a category:

//Removing previous categories
$previousCategoryIds = $product->getCategoryIds();
foreach($previousCategoryIds as $k=>$previousCategoryId){
    $this->categoryLinkRepository->deleteByIds($previousCategoryId,$product->getSku());
}

//Assigning new categories
$this->categoryLinkManagement->assignProductToCategories($product->getSku(),$categories);

The code is fine, but I am getting this error:

Fatal error: Uncaught Error: Call to undefined method Magento\Catalog\Model\ResourceModel\Category\Flat::saveAttribute() in ...vendor/magento/module-catalog-url-rewrite/Observer/CategoryUrlPathAutogeneratorObserver.php:96

I have searched this error and found the cause of this error:
https://github.com/magento/magento2/issues/9725#issuecomment-346621159

The problem is, category link repository and link management is only for backend, and I am trying to use it on frontend scope that's why it is causing this error.

I have also tried this:

$product->setCategoryIds($categories);

But above code appending categories, not replacing previous categories.

Can you help me resolve this issue? I have tried everything I have found.

Edit:

The problem is with flat category, if I enable flat category then error occurs, and if I disable flat category it is working fine.

Answer 1

can you try this code it's works for me

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();

$CategoryLinkRepository = $objectManager->get('\Magento\Catalog\Model\CategoryLinkRepository');

$categoryId = 4;
$sku = 'Jacket';

$CategoryLinkRepository->deleteByIds($categoryId,$sku);

$categoryIds = [3]; // your category ids Array like [2,3,5]
$product = $objectManager->get('Magento\Catalog\Model\Product')->loadByAttribute('sku', $sku);
$product->setCategoryIds($categoryIds);
$product->save();