How to use custom product attribute in sorting option with search criteria builder for magento 2?

Question

I have created one custom product attribute(bestseller). Now I want to sort by custom product attribute(bestseller) using search criteria builder sort by option but it's not working for custom product attribute. FYI Sort by option working with price attribute but not working with custom product attribute option. Any help appreciate...

//Code for fetch products based on search term.
$limit = $this->scopeConfig->getValue('catalog/frontend/list_per_page');    
$p = $p?$p:1;
$pageSize = $limit?$limit:20;            
$this->_storeManager->setCurrentStore($storeid);
$search_criteria = $this->searchCriteriaBuilder->create();
$search_criteria->setRequestName("quick_search_container");
$search_criteria->setCurrentPage($p);
$search_criteria->setPageSize($pageSize);
$filter = $this->filterBuilder->setField('search_term')
->setValue($q)
->setConditionType("like")
->create();               
$filterGroup = $this->filterGroupBuilder->addFilter($filter)->create();
$search_criteria->setFilterGroups([$filterGroup]);
$sortOrder = $this->sortOrderBuilder->setField('bestseller')->setDirection(SortOrder::SORT_DESC)->create();
$search_criteria->setSortOrders([$sortOrder]);
$search_result = $this->searchInterface->search($search_criteria);
$totalSearchResults = $search_result->getTotalCount();        
$products = $search_result->getItems();

Answer 1

Inject 3 classes

use Magento\Framework\Api\SearchCriteriaBuilder;
use Magento\Framework\Api\SearchCriteriaInterface;
use Magento\Framework\Api\SearchCriteria\CollectionProcessorInterface;

check below code

   public function getList(SearchCriteriaInterface $searchCriteria)
    {
        /** @var \Custom\Module\Model\ResourceModel\CustomModel\CollectionFactory $collection */
        $collection = $this->CustomModelListCollectionFactory->create();
        foreach ($searchCriteria->getFilterGroups() as $filterGroup) {
            $fields = [];
            $conditions = [];
            foreach ($filterGroup->getFilters() as $filter) {
                if ($filter->getField() === 'store_id') {
                    $collection->addStoreFilter($filter->getValue(), false);
                    continue;
                }
                $fields[] = $filter->getField();
                $condition = $filter->getConditionType() ?: 'eq';
                $conditions[] = [$condition => $filter->getValue()];
            }
            $collection->addFieldToFilter($fields, $conditions);
        }

        $sortOrders = $searchCriteria->getSortOrders();
        if ($sortOrders) {
            /** @var SortOrder $sortOrder */
            foreach ($sortOrders as $sortOrder) {
                $collection->addOrder(
                    $sortOrder->getField(),
                    ($sortOrder->getDirection() === SortOrder::SORT_ASC) ? 'ASC' : 'DESC'
                );
            }
        }
        $collection->setCurPage($searchCriteria->getCurrentPage());
        $collection->setPageSize($searchCriteria->getPageSize());
        $this->collectionProcessor->process($searchCriteria, $collection);

        /** @var \Custom\Module\Api\Data\CustomModelSearchResultsInterfaceFactory $searchResults */
        $searchResults = $this->searchResultsFactory->create();
        $searchResults->setSearchCriteria($searchCriteria);
        $searchResults->setTotalCount($collection->getSize());
        $searchResults->setItems($collection->getItems());

        return $searchResults;
    }

below sort condition

$sortOrders = $searchCriteria->getSortOrders();
        if ($sortOrders) {
            /** @var SortOrder $sortOrder */
            foreach ($sortOrders as $sortOrder) {
                $collection->addOrder(
                    $sortOrder->getField(),
                    ($sortOrder->getDirection() === SortOrder::SORT_ASC) ? 'ASC' : 'DESC'
                );
            }
        }