If your extra data directly references a post meta you don’t have to do anything, because WP_Post implements the »magic« methods __isset() and __get() which directly asks for post meta keys (except for the following four keys: page_template, post_category, tags_input and ancestors). Here’s a quick example that shows the behavior:
<?php
$post_id = 42;
$meta_key = 'extra_data_1';
add_post_meta( $post_id, $meta_key, [ 'some', 'value' ], TRUE );
$post = get_post( $post_id );
var_dump( $post->{$meta_key} ); // (array) ['some', 'value']
In any other case, use the filter posts_results:
<?php
add_filter(
'posts_results',
function( array $posts, WP_Query $query ) {
foreach ( $posts as $post ) {
$post->extra_data_1 = get_post_meta( $post->ID, 'extra_data' );
// and so on …
}
return $posts;
},
10,
2
);
However I would suggest to use an object oriented approach and create own entity interfaces, that follows your problem domain. The implementations then wrapping WP_Post instances as a dependency. For example, let’s say you’re dealing with books:
<?php
namespace Wpse240042\Type;
use WP_Post;
interface Book {
/**
* @return string
*/
public function title();
/**
* @return string
*/
public function publisher();
/**
* @return int
*/
public function year();
}
class WpPostBook implements Book {
/**
* @var WP_Post
*/
private $post;
/**
* @param WP_Post $post
*/
public function __construct( WP_Post $post ) {
$this->post = $post;
}
/**
* @return string
*/
public function title() {
return $this->post->post_title;
}
/**
* @return string
*/
public function publisher() {
return get_post_meta( $this->post->ID, '_book_publisher', TRUE );
}
/**
* @return int
*/
public function year() {
return get_post_meta( $this->post->ID, '_book_publisher', TRUE );
}
}
Your business logic can then rely on the structure of a book by type hint the type Book on every dependency. To fetch a list of books you could implement a factory in the first step that wraps a WP_Query or fetch WP_Query arguments and return a list of book instances. You should not use the posts_results filter in that case to replace WP_Query::posts with a list of Type\Book instances to not break type consistency throughout WP core.