Prioritise core WooCommerce GTIN field in product adapter

src/Product/WCProductAdapter.php

@@ -109,6 +109,7 @@ public function mapTypes( $properties ) {
 		$this->map_woocommerce_product();
 		$this->map_attribute_mapping_rules( $mapping_rules );
 		$this->map_gla_attributes( $gla_attributes );
+		$this->map_gtin();
 
 		// Allow users to override the product's attributes using a WordPress filter.
 		$this->override_attributes();
@@ -927,6 +928,28 @@ protected function map_gla_attributes( array $attributes ): WCProductAdapter {
 		return $this;
 	}
 
+	/**
+	 * Map the WooCommerce core global unique ID (GTIN) value if it's available.
+	 *
+	 * @since x.x.x
+	 *
+	 * @return $this
+	 */
+	protected function map_gtin(): WCProductAdapter {
+		// compatibility-code "WC < 9.2" -- Core global unique ID field was added in 9.2
+		if ( ! method_exists( $this->wc_product, 'get_global_unique_id' ) ) {
+			return $this;
+		}
+
+		$global_unique_id = $this->wc_product->get_global_unique_id();
+
+		if ( ! empty( $global_unique_id ) ) {
+			$this->setGtin( $global_unique_id );
+		}
+
+		return $this;
+	}
+
 	/**
 	 * @param string $targetCountry
 	 *

tests/Unit/Product/Attributes/GtinMappingTest.php

@@ -0,0 +1,131 @@
+<?php
+declare( strict_types=1 );
+
+namespace Automattic\WooCommerce\GoogleListingsAndAds\Tests\Unit\Product\Attributes;
+
+use Automattic\WooCommerce\GoogleListingsAndAds\Product\WCProductAdapter;
+use PHPUnit\Framework\TestCase;
+use WC_Helper_Product;
+
+/**
+ * Class GtinMappingTest
+ *
+ * Unit tests to confirm that the GTIN value is mapped correctly.
+ * The value should be prioritised in the following order:
+ *
+ * 1. WooCommerce Core:       Global Unique ID
+ * 2. Google for WooCommerce: GTIN attribute
+ * 3. Google for WooCommerce: Attribute mapping rules
+ */
+class GtinMappingTest extends TestCase {
+
+	/** @var string $core_gtin Mock value to be used as the core gtin field. */
+	private $core_gtin = '219837492834';
+
+	/** @var string $gla_gtin Mock value to be used as the Google for WooCommerce gtin field. */
+	private $gla_gtin = 'gla-gtin-field';
+
+	/** @var string $gla_attribute_mapping_gtin Mock value to be used as the Google for WooCommerce attribute mapping gtin value. */
+	private $gla_attribute_mapping_gtin = 'gla-attribute-mapping-gtin';
+
+	/**
+	 * Test GTIN mapping from WooCommerce Core Global Unique ID.
+	 *
+	 * @return void
+	 */
+	public function test_gtin_populated_from_wc_core_global_unique_id() {
+		$mock_product = WC_Helper_Product::create_simple_product( false );
+		$mock_product->set_global_unique_id( $this->core_gtin );
+
+		$adapter = new WCProductAdapter();
+		$adapter->mapTypes(
+			[
+				'wc_product'     => $mock_product,
+				'targetCountry'  => 'US',
+				'gla_attributes' => [
+					'gtin' => $this->gla_gtin,
+				],
+			]
+		);
+
+		$this->assertEquals( $this->core_gtin, $adapter->getGtin() );
+	}
+
+	/**
+	 * Test GTIN mapping from Google for WooCommerce GTIN attribute.
+	 *
+	 * @return void
+	 */
+	public function test_gtin_populated_from_gla_gtin_attribute() {
+		$mock_product = WC_Helper_Product::create_simple_product( false );
+		$mock_product->set_sku( $this->gla_attribute_mapping_gtin );
+
+		$adapter = new WCProductAdapter();
+		$adapter->mapTypes(
+			[
+				'wc_product'     => $mock_product,
+				'targetCountry'  => 'US',
+				'mapping_rules'  => [
+					[
+						'attribute'               => 'gtin',
+						'source'                  => 'product:sku',
+						'category_condition_type' => 'all',
+						'categories'              => '',
+					],
+				],
+				'gla_attributes' => [
+					'gtin' => $this->gla_gtin,
+				],
+			]
+		);
+
+		$this->assertEquals( $this->gla_gtin, $adapter->getGtin() );
+	}
+
+	/**
+	 * Test GTIN mapping from Google for WooCommerce attribute mapping rules.
+	 *
+	 * @return void
+	 */
+	public function test_gtin_populated_from_attribute_mapping_rules() {
+		$mock_product = WC_Helper_Product::create_simple_product( false );
+		$mock_product->set_sku( $this->gla_attribute_mapping_gtin );
+
+		$adapter = new WCProductAdapter();
+		$adapter->mapTypes(
+			[
+				'wc_product'    => $mock_product,
+				'targetCountry' => 'US',
+				'mapping_rules' => [
+					[
+						'attribute'               => 'gtin',
+						'source'                  => 'product:sku',
+						'category_condition_type' => 'all',
+						'categories'              => '',
+					],
+				],
+			]
+		);
+
+		$this->assertEquals( $this->gla_attribute_mapping_gtin, $adapter->getGtin() );
+	}
+
+	/**
+	 * Test GTIN remains empty when no data is available.
+	 *
+	 * @return void
+	 */
+	public function test_gtin_remains_empty_when_no_data_available() {
+		$mock_product = WC_Helper_Product::create_simple_product( false );
+
+		$adapter = new WCProductAdapter();
+		$adapter->mapTypes(
+			[
+				'wc_product'    => $mock_product,
+				'targetCountry' => 'US',
+			]
+		);
+
+		$this->assertEquals( '', $adapter->getGtin() );
+	}
+}