FSE: Strip post ids from template part blocks on export.

lib/edit-site-export.php

@@ -28,7 +28,10 @@ function gutenberg_edit_site_export() {
 
 		$current_template = gutenberg_find_template_post_and_parts( $template_type );
 		if ( isset( $current_template ) ) {
-			$zip->addFromString( 'theme/block-templates/' . $current_template['template_post']->post_name . '.html', $current_template['template_post']->post_content );
+			$zip->addFromString(
+				'theme/block-templates/' . $current_template['template_post']->post_name . '.html',
+				gutenberg_strip_post_ids_from_template_part_blocks( $current_template['template_post']->post_content )
+			);
 
 			foreach ( $current_template['template_part_ids'] as $template_part_id ) {
 				$template_part = get_post( $template_part_id );
@@ -62,3 +65,29 @@ function () {
 		);
 	}
 );
+
+/**
+ * Remove post id attributes from template part blocks.
+ *
+ * This is needed so that Gutenberg loads the HTML file of the template, instead of looking for a template part post.
+ *
+ * @param string $template_content Template content to modify.
+ *
+ * @return string Potentially modified template content.
+ */
+function gutenberg_strip_post_ids_from_template_part_blocks( $template_content ) {
+	$blocks = parse_blocks( $template_content );
+
+	array_walk(
+		$blocks,
+		function( &$block ) {
+			if ( 'core/template-part' !== $block['blockName'] ) {
+				return;
+			}
+
+			unset( $block['attrs']['postId'] );
+		}
+	);
+
+	return serialize_blocks( $blocks );
+}

phpunit/bootstrap.php

@@ -73,6 +73,7 @@ function fail_if_died( $message ) {
 $GLOBALS['wp_tests_options'] = array(
 	'gutenberg-experiments' => array(
 		'gutenberg-widget-experiments' => '1',
+		'gutenberg-full-site-editing'  => 1,
 	),
 );
 

phpunit/class-edit-site-export-test.php

@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * Test `gutenberg_edit_site_export` and its helper functions.
+ *
+ * @package Gutenberg
+ */
+
+class Edit_Site_Export_Test extends WP_UnitTestCase {
+	/**
+	 * Verify that post ids are stripped from template part blocks during the export.
+	 *
+	 * This is needed so that the exported template is loaded from the theme.
+	 */
+	public function test_post_ids_are_stripped_from_template_parts() {
+		$post = self::factory()->post->create_and_get(
+			array(
+				'post_content' => '<!-- wp:template-part {"postId":123,"slug":"header","theme":"gutenberg-tests"} /-->',
+				'post_title'   => 'Archive',
+				'post_type'    => 'wp_template',
+			)
+		);
+
+		$this->assertSame(
+			'<!-- wp:template-part {"slug":"header","theme":"gutenberg-tests"} /-->',
+			gutenberg_strip_post_ids_from_template_part_blocks( $post->post_content )
+		);
+	}
+}