Latest Posts: Fix selected category on existing blocks

packages/block-library/src/latest-posts/block.json

@@ -10,7 +10,10 @@
 			"type": "string"
 		},
 		"categories": {
-			"type": "array"
+			"type": "array",
+			"items": {
+				"type": "object"
+			}
 		},
 		"postsToShow": {
 			"type": "number",

packages/block-library/src/latest-posts/deprecated.js

@@ -0,0 +1,31 @@
+/**
+ * Internal dependencies
+ */
+import metadata from './block.json';
+
+const { attributes } = metadata;
+
+export default [
+	{
+		attributes: {
+			...attributes,
+			categories: {
+				type: 'string',
+			},
+		},
+		supports: {
+			align: true,
+			html: false,
+		},
+		migrate: ( oldAttributes ) => {
+			// This needs the full category object, not just the ID.
+			return {
+				...oldAttributes,
+				categories: [ { id: Number( oldAttributes.categories ) } ],
+			};
+		},
+		isEligible: ( { categories } ) =>
+			categories && 'string' === typeof categories,
+		save: () => null,
+	},
+];

packages/block-library/src/latest-posts/edit.js

@@ -104,13 +104,6 @@ class LatestPostsEdit extends Component {
 			featuredImageSizeWidth,
 			featuredImageSizeHeight,
 		} = attributes;
-		const suggestions = categoriesList.reduce(
-			( accumulator, category ) => ( {
-				...accumulator,
-				[ category.name ]: category,
-			} ),
-			{}
-		);
 		const categorySuggestions = categoriesList.reduce(
 			( accumulator, category ) => ( {
 				...accumulator,
@@ -120,15 +113,18 @@ class LatestPostsEdit extends Component {
 		);
 		const selectCategories = ( tokens ) => {
 			const hasNoSuggestion = tokens.some(
-				( token ) => typeof token === 'string' && ! suggestions[ token ]
+				( token ) =>
+					typeof token === 'string' && ! categorySuggestions[ token ]
 			);
 			if ( hasNoSuggestion ) {
 				return;
 			}
 			// Categories that are already will be objects, while new additions will be strings (the name).
 			// allCategories nomalizes the array so that they are all objects.
 			const allCategories = tokens.map( ( token ) => {
-				return typeof token === 'string' ? suggestions[ token ] : token;
+				return typeof token === 'string'
+					? categorySuggestions[ token ]
+					: token;
 			} );
 			// We do nothing if the category is not selected
 			// from suggestions.

packages/block-library/src/latest-posts/index.js

@@ -7,6 +7,7 @@ import { postList as icon } from '@wordpress/icons';
 /**
  * Internal dependencies
  */
+import deprecated from './deprecated';
 import edit from './edit';
 import metadata from './block.json';
 
@@ -23,4 +24,5 @@ export const settings = {
 		html: false,
 	},
 	edit,
+	deprecated,
 };

packages/block-library/src/latest-posts/index.php

@@ -173,3 +173,34 @@ function register_block_core_latest_posts() {
 	);
 }
 add_action( 'init', 'register_block_core_latest_posts' );
+
+/**
+ * Handles outdated versions of the `core/latest-posts` block by converting
+ * attribute `categories` from a numeric string to an array with key `id`.
+ *
+ * This is done to accommodate the changes introduced in #20781 that sought to
+ * add support for multiple categories to the block. However, given that this
+ * block is dynamic, the usual provisions for block migration are insufficient,
+ * as they only act when a block is loaded in the editor.
+ *
+ * TODO: Remove when and if the bottom client-side deprecation for this block
+ * is removed.
+ *
+ * @param array $block A single parsed block object.
+ *
+ * @return array The migrated block object.
+ */
+function block_core_latest_posts_migrate_categories( $block ) {
+	if (
+		'core/latest-posts' === $block['blockName'] &&
+		! empty( $block['attrs']['categories'] ) &&
+		is_string( $block['attrs']['categories'] )
+	) {
+		$block['attrs']['categories'] = array(
+			array( 'id' => absint( $block['attrs']['categories'] ) ),
+		);
+	}
+
+	return $block;
+}
+add_filter( 'render_block_data', 'block_core_latest_posts_migrate_categories' );