Add new GraphQL query for single product retrieval

app/code/Magento/CatalogGraphQl/Model/Resolver/Product/DataProvider/ProductDataProvider.php

@@ -0,0 +1,58 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\CatalogGraphQl\Model\Resolver\Product\DataProvider;
+
+use Magento\Catalog\Api\Data\ProductInterface;
+use Magento\Catalog\Model\ProductFactory;
+use Magento\Catalog\Model\ResourceModel\Product as ProductResourceModel;
+
+/**
+ * Product data provider, used for GraphQL resolver processing.
+ */
+class ProductDataProvider implements ProductDataProviderInterface
+{
+    /**
+     * @var ProductResourceModel
+     */
+    private $productResourceModel;
+
+    /**
+     * @var ProductFactory
+     */
+    private $productFactory;
+
+    /**
+     * ProductDataProvider constructor.
+     *
+     * @param ProductResourceModel $productResourceModel
+     * @param ProductFactory $productFactory
+     */
+    public function __construct(
+        ProductResourceModel $productResourceModel,
+        ProductFactory $productFactory
+    ) {
+        $this->productResourceModel = $productResourceModel;
+        $this->productFactory = $productFactory;
+    }
+
+    /**
+     * Get product data by ID with full data set
+     *
+     * @param int $productId
+     * @param array $attributeCodes
+     * @return ProductInterface|Product
+     */
+    public function getProductById(int $productId, array $attributeCodes)
+    {
+        /** @var \Magento\Catalog\Model\Product $product */
+        $product = $this->productFactory->create();
+        $this->productResourceModel->load($product, $productId, $attributeCodes);
+
+        return $product;
+    }
+}

app/code/Magento/CatalogGraphQl/Model/Resolver/Product/DataProvider/ProductDataProviderInterface.php

@@ -0,0 +1,25 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\CatalogGraphQl\Model\Resolver\Product\DataProvider;
+
+use Magento\Catalog\Api\Data\ProductInterface;
+
+/**
+ * Provides product data by product ID.
+ */
+interface ProductDataProviderInterface
+{
+    /**
+     * Retrieve product by product ID.
+     *
+     * @param int $productId
+     * @param array $attributeCodes
+     * @return ProductInterface
+     */
+    public function getProductById(int $productId, array $attributeCodes);
+}

app/code/Magento/CatalogGraphQl/Model/Resolver/Product/ProductIdIdentity.php

@@ -0,0 +1,34 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\CatalogGraphQl\Model\Resolver\Product;
+
+use Magento\Catalog\Model\Product;
+use Magento\Framework\GraphQl\Query\Resolver\IdentityInterface;
+
+/**
+ * Identity for resolved product by ID
+ */
+class ProductIdIdentity implements IdentityInterface
+{
+    /**
+     * @var string
+     */
+    private $cacheTag = Product::CACHE_TAG;
+
+    /**
+     * Get product id for cache tag
+     *
+     * @param array $resolvedData
+     * @return array
+     */
+    public function getIdentities(array $resolvedData): array
+    {
+        return empty($resolvedData['id']) ?
+            [] : [$this->cacheTag, $resolvedData['id']];
+    }
+}

app/code/Magento/CatalogGraphQl/Model/Resolver/ProductId.php

@@ -0,0 +1,72 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\CatalogGraphQl\Model\Resolver;
+
+use Magento\CatalogGraphQl\Model\Resolver\Product\DataProvider\ProductDataProviderInterface;
+use Magento\CatalogGraphQl\Model\Resolver\Product\ProductFieldsSelector;
+use Magento\Framework\GraphQl\Config\Element\Field;
+use Magento\Framework\GraphQl\Exception\GraphQlInputException;
+use Magento\Framework\GraphQl\Query\ResolverInterface;
+use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
+
+/**
+ * Product by ID resolver, used for GraphQL request processing.
+ */
+class ProductId implements ResolverInterface
+{
+    /**
+     * @var ProductDataProviderInterface
+     */
+    private $productDataProvider;
+
+    /**
+     * @var ProductFieldsSelector
+     */
+    private $productFieldsSelector;
+
+    /**
+     * ProductId constructor.
+     *
+     * @param ProductDataProviderInterface $productDataProvider
+     * @param ProductFieldsSelector $productFieldsSelector
+     */
+    public function __construct(
+        ProductDataProviderInterface $productDataProvider,
+        ProductFieldsSelector $productFieldsSelector
+    ) {
+        $this->productDataProvider = $productDataProvider;
+        $this->productFieldsSelector = $productFieldsSelector;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function resolve(
+        Field $field,
+        $context,
+        ResolveInfo $info,
+        array $value = null,
+        array $args = null
+    ) {
+        $productId = (int) ($args['id'] ?? 0);
+        $fields = $this->productFieldsSelector->getProductFieldsFromInfo($info);
+
+        if (!$productId) {
+            throw new GraphQlInputException(
+                __("'id' input argument is required.")
+            );
+        }
+
+        /** @var \Magento\Catalog\Model\Product $product */
+        $product = $this->productDataProvider->getProductById($productId, $fields);
+        $data = $product->getData();
+        $data['model'] = $product;
+
+        return $data;
+    }
+}

app/code/Magento/CatalogGraphQl/etc/di.xml

@@ -74,4 +74,5 @@
     <preference type="\Magento\CatalogGraphQl\Model\Resolver\Product\Price\Provider" for="\Magento\CatalogGraphQl\Model\Resolver\Product\Price\ProviderInterface"/>
 
     <preference type="Magento\CatalogGraphQl\Model\Resolver\Products\Query\Search" for="Magento\CatalogGraphQl\Model\Resolver\Products\Query\ProductQueryInterface"/>
+    <preference type="Magento\CatalogGraphQl\Model\Resolver\Product\DataProvider\ProductDataProvider" for="Magento\CatalogGraphQl\Model\Resolver\Product\DataProvider\ProductDataProviderInterface"/>
 </config>

app/code/Magento/CatalogGraphQl/etc/schema.graphqls

@@ -10,6 +10,10 @@ type Query {
         sort: ProductAttributeSortInput @doc(description: "Specifies which attributes to sort on, and whether to return the results in ascending or descending order.")
     ): Products
     @resolver(class: "Magento\\CatalogGraphQl\\Model\\Resolver\\Products") @doc(description: "The products query searches for products that match the criteria specified in the search and filter attributes.") @cache(cacheIdentity: "Magento\\CatalogGraphQl\\Model\\Resolver\\Product\\Identity")
+    product (
+        id: Int @doc(description: "Id of the product.")
+    ): ProductInterface
+    @resolver(class: "Magento\\CatalogGraphQl\\Model\\Resolver\\ProductId") @doc(description: "The product query searches for product that match the criteria specified in the search and filter attributes.") @cache(cacheIdentity: "Magento\\CatalogGraphQl\\Model\\Resolver\\Product\\ProductIdIdentity")
     category (
      id: Int @doc(description: "Id of the category.")
     ): CategoryTree