* Meta API: WP_Metadata_Lazyloader class
* Core class used for lazy-loading object metadata.
* When loading many objects of a given type, such as posts in a WP_Query loop, it often makes
* sense to prime various metadata caches at the beginning of the loop. This means fetching all
* relevant metadata with a single database query, a technique that has the potential to improve
* performance dramatically in some cases.
* In cases where the given metadata may not even be used in the loop, we can improve performance
* even more by only priming the metadata cache for affected items the first time a piece of metadata
* is requested - ie, by lazy-loading it. So, for example, comment meta may not be loaded into the
* cache in the comments section of a post until the first time get_comment_meta() is called in the
* context of the comment loop.
* WP uses the WP_Metadata_Lazyloader class to queue objects for metadata cache priming. The class
* then detects the relevant get_*_meta() function call, and queries the metadata of all queued objects.
* Do not access this class directly. Use the wp_metadata_lazyloader() function.
#[AllowDynamicProperties]
class WP_Metadata_Lazyloader {
protected $pending_objects;
* Settings for supported object types.
protected $settings = array();
public function __construct() {
'filter' => 'get_term_metadata',
'callback' => array( $this, 'lazyload_meta_callback' ),
'filter' => 'get_comment_metadata',
'callback' => array( $this, 'lazyload_meta_callback' ),
'filter' => 'get_blog_metadata',
'callback' => array( $this, 'lazyload_meta_callback' ),
* Adds objects to the metadata lazy-load queue.
* @param string $object_type Type of object whose meta is to be lazy-loaded. Accepts 'term' or 'comment'.
* @param array $object_ids Array of object IDs.
* @return void|WP_Error WP_Error on failure.
public function queue_objects( $object_type, $object_ids ) {
if ( ! isset( $this->settings[ $object_type ] ) ) {
return new WP_Error( 'invalid_object_type', __( 'Invalid object type.' ) );
$type_settings = $this->settings[ $object_type ];
if ( ! isset( $this->pending_objects[ $object_type ] ) ) {
$this->pending_objects[ $object_type ] = array();
foreach ( $object_ids as $object_id ) {
// Keyed by ID for faster lookup.
if ( ! isset( $this->pending_objects[ $object_type ][ $object_id ] ) ) {
$this->pending_objects[ $object_type ][ $object_id ] = 1;
add_filter( $type_settings['filter'], $type_settings['callback'], 10, 5 );
* Fires after objects are added to the metadata lazy-load queue.
* @param array $object_ids Array of object IDs.
* @param string $object_type Type of object being queued.
* @param WP_Metadata_Lazyloader $lazyloader The lazy-loader object.
do_action( 'metadata_lazyloader_queued_objects', $object_ids, $object_type, $this );
* Resets lazy-load queue for a given object type.
* @param string $object_type Object type. Accepts 'comment' or 'term'.
* @return void|WP_Error WP_Error on failure.
public function reset_queue( $object_type ) {
if ( ! isset( $this->settings[ $object_type ] ) ) {
return new WP_Error( 'invalid_object_type', __( 'Invalid object type.' ) );
$type_settings = $this->settings[ $object_type ];
$this->pending_objects[ $object_type ] = array();
remove_filter( $type_settings['filter'], $type_settings['callback'] );
* Lazy-loads term meta for queued terms.
* This method is public so that it can be used as a filter callback. As a rule, there
* is no need to invoke it directly.
* @deprecated 6.3.0 Use WP_Metadata_Lazyloader::lazyload_meta_callback() instead.
* @param mixed $check The `$check` param passed from the 'get_term_metadata' hook.
* @return mixed In order not to short-circuit `get_metadata()`. Generally, this is `null`, but it could be
* another value if filtered by a plugin.
public function lazyload_term_meta( $check ) {
_deprecated_function( __METHOD__, '6.3.0', 'WP_Metadata_Lazyloader::lazyload_meta_callback' );
return $this->lazyload_meta_callback( $check, 0, '', false, 'term' );
* Lazy-loads comment meta for queued comments.
* This method is public so that it can be used as a filter callback. As a rule, there is no need to invoke it
* directly, from either inside or outside the `WP_Query` object.
* @deprecated 6.3.0 Use WP_Metadata_Lazyloader::lazyload_meta_callback() instead.
* @param mixed $check The `$check` param passed from the {@see 'get_comment_metadata'} hook.
* @return mixed The original value of `$check`, so as not to short-circuit `get_comment_metadata()`.
public function lazyload_comment_meta( $check ) {
_deprecated_function( __METHOD__, '6.3.0', 'WP_Metadata_Lazyloader::lazyload_meta_callback' );
return $this->lazyload_meta_callback( $check, 0, '', false, 'comment' );
* Lazy-loads meta for queued objects.
* This method is public so that it can be used as a filter callback. As a rule, there
* is no need to invoke it directly.
* @param mixed $check The `$check` param passed from the 'get_*_metadata' hook.
* @param int $object_id ID of the object metadata is for.
* @param string $meta_key Unused.
* @param bool $single Unused.
* @param string $meta_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
* or any other object type with an associated meta table.
* @return mixed In order not to short-circuit `get_metadata()`. Generally, this is `null`, but it could be
* another value if filtered by a plugin.
public function lazyload_meta_callback( $check, $object_id, $meta_key, $single, $meta_type ) {
if ( empty( $this->pending_objects[ $meta_type ] ) ) {
$object_ids = array_keys( $this->pending_objects[ $meta_type ] );
if ( $object_id && ! in_array( $object_id, $object_ids, true ) ) {
$object_ids[] = $object_id;
update_meta_cache( $meta_type, $object_ids );
// No need to run again for this set of objects.
$this->reset_queue( $meta_type );
It is recommended that you Edit text format, this type of Fix handles quite a lot in one request
