Elasticsearch API Platform -> FOS Elastica configuration

ElasticsearchProductProvider.php

<?php

namespace App\Search;

use ApiPlatform\Core\DataProvider;
use FOS\ElasticaBundle\Index\IndexManager;
use Symfony\Component\Serializer\Serializer;

/**
 * This class is based on API Platforms elasticsearch implemenation.
 * However, It uses the FOS Elastica Bundle connection.
 *
 * @see \ApiPlatform\Core\Bridge\Elasticsearch\DataProvider\CollectionDataProvider
 * @see \ApiPlatform\Core\Bridge\Elasticsearch\DataProvider\ItemDataProvider
 */
class ElasticsearchProductProvider implements DataProvider\ContextAwareCollectionDataProviderInterface, DataProvider\ItemDataProviderInterface, DataProvider\RestrictedDataProviderInterface
{
    private const INDEX_NAME = 'product';

    private IndexManager $indexManager;
    private DataProvider\Pagination $pagination;
    private Serializer $serializer;

    public function __construct(IndexManager $indexManager, DataProvider\Pagination $pagination, Serializer $serializer)
    {
        $this->indexManager = $indexManager;
        $this->pagination = $pagination;
        $this->serializer = $serializer;
    }

    public function supports(string $resourceClass, string $operationName = null, array $context = []): bool
    {
        return $resourceClass === \App\Entity\Product::class;
    }

    public function getItem(string $resourceClass, $id, string $operationName = null, array $context = [])
    {
        $query = [
            'query' => [['term' => ['id' => $id]]],
        ];

        $index = $this->indexManager->getIndex(self::INDEX_NAME);
        $response = $index->request('_search', 'GET', $query)->getData();
        if (count($response['hits']['hits']) < 1) {
            return null;
        }

        return $this->serializer->denormalize(
            $response['hits']['hits'][0]['_source'],
            $resourceClass
        );
    }

    public function getCollection(string $resourceClass, string $operationName = null, array $context = [])
    {
        $query = [
            'from' => $this->pagination->getOffset($resourceClass, $operationName, $context),
            'size' => $this->pagination->getLimit($resourceClass, $operationName, $context),
        ];

        // $context['filters'] has the query string, you can use that to build the query
        // you can also build filter classes with \ApiPlatform\Core\Api\FilterInterface
        // look at existing filters to figure out how api platform intents this to be used

        $index = $this->indexManager->getIndex(self::INDEX_NAME);
        $response = $index->request('_search', 'GET', $query)->getData();

        return new \ApiPlatform\Core\Bridge\Elasticsearch\DataProvider\Paginator(
            $this->serializer,
            $response,
            $resourceClass,
            $query['size'],
            $query['from']
        );
    }
}

fos_elastica.yaml

parameters:
    env(ELASTICSEARCH_VERIFYPEER): yes
    env(ELASTICSEARCH_VERIFYHOST): 2
    
fos_elastica:
    
    # the database connection
    clients:
        default:
            url: '%env(ELASTICSEARCH_URL)%'
            curl:
                # with this, you can disable ssl host check, which is useful in dev environments
                !php/const CURLOPT_SSL_VERIFYPEER: '%env(bool:ELASTICSEARCH_VERIFYPEER)%'
                !php/const CURLOPT_SSL_VERIFYHOST: '%env(int:ELASTICSEARCH_VERIFYHOST)%'
                
    indexes:
        product:
            persistence:
                driver: orm
                model: App\Entity\Product
                provider: ~ # you can set a provider to build your model yourself
                finder: ~
                
            # this defines how the domain model is serialized before being commited to elasticsearch
            # https://symfony.com/doc/5.4/serializer.html#using-serialization-groups-annotations
            serializer:
                groups: [search]
            
            # these properties are 1:1 the properties for elasticsearchs explicit mapping
            # https://www.elastic.co/guide/en/elasticsearch/reference/7.17/explicit-mapping.html
            # this, unintuitively, has nothing to do with serialization
            properties:
                title: { type: text, boost: 2.0 }

FulltextFilter.php

<?php

namespace App\ApiPlatform\Filter;

use ApiPlatform\Core\Bridge\Doctrine\Orm\Filter\ContextAwareFilterInterface;
use ApiPlatform\Core\Bridge\Doctrine\Orm\Util\QueryNameGeneratorInterface;
use Doctrine\ORM\QueryBuilder;
use FOS\ElasticaBundle\Index\IndexManager;

class FulltextFilter implements ContextAwareFilterInterface
{
    private const INDEX_NAME = 'product';
    private IndexManager $indexManager;

    public function __construct(IndexManager $indexManager)
    {
        $this->indexManager = $indexManager;
    }

    public function getDescription(string $resourceClass): array
    {
        return [
            'q' => [
                'property' => 'q',
                'type' => 'string',
                'required' => false,
            ],
            'order[relevance]' => [
                'property' => 'order[relevance]',
                'type' => 'string',
                'required' => false,
            ],
        ];
    }

    public function apply(QueryBuilder $queryBuilder, QueryNameGeneratorInterface $queryNameGenerator, string $resourceClass, string $operationName = null, array $context = [])
    {
        $query = [
            '_source' => false, // you don't need the source document, just the ids
            'size' => 10000, // elasticsearch limit per page
            'query' => [
                'multi_match' => [
                    'query' => $context['filters']['query'],
                    'fields' => array_keys($this->properties),
                ]
            ],
        ];

        $response = $this->indexManager->getIndex(self::INDEX_NAME)->request('/_search', 'GET', $query);
        $ids = array_column($response->getData()['hits']['hits'] ?? [], '_id');

        if (empty($ids)) {
            $queryBuilder->andWhere('1 = 0'); // enforce empty result
            return;
        }

        // search for the ids in the query
        $rootAlias = $queryBuilder->getRootAliases()[0];
        $queryBuilder->andWhere("$rootAlias.id IN (:fulltext_search_filter_ids)");
        $queryBuilder->setParameter('fulltext_search_filter_ids', $ids);

        // sort by id list aka relevance
        if (in_array($context['filters']['order']['relevance'] ?? null, ['asc', 'desc'], true)) {
            $queryBuilder->addSelect("FIELD($rootAlias.id, :fulltext_search_filter_ids) AS HIDDEN __order");
            $queryBuilder->addOrderBy("__order", $context['filters']['order']['relevance']);
        }
    }
}