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metadata_exists()

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概述

metadata_exists() 函数用于检查指定对象 ID 的元数据字段是否存在。它支持多种对象类型，如 'post'、'user'、'term' 等，并利用缓存机制优化性能。

关键要点

  • 函数签名：metadata_exists( $meta_type, $object_id, $meta_key )，返回布尔值表示元字段是否存在
  • 参数要求：$meta_type 必须为字符串（如 'blog'、'post'、'comment'、'term'、'user'），$object_id 必须为数字，$meta_key 必须为字符串
  • 内部流程：先应用 get_{$meta_type}_metadata 过滤器，然后检查缓存，必要时更新缓存，最后判断元键是否存在
  • 相关函数：与 update_meta_cache()、wp_cache_get()、absint()、apply_filters() 等协同工作
  • 使用场景：常用于在获取元数据前检查其存在性，避免不必要的数据库查询

代码示例

// Check and get a term meta
if ( metadata_exists( 'term', $term_id, '_meta_key' ) ) {
    $meta_value = get_term_meta( $term_id, '_meta_key', true );
}

// Check and get a post meta
if ( metadata_exists( 'post', $post_id, '_meta_key' ) ) {
    $meta_value = get_post_meta( $post_id, '_meta_key', true );
}

// Check and get a user meta
if ( metadata_exists( 'user', $user_id, '_meta_key' ) ) {
    $meta_value = get_user_meta( $user_id, '_meta_key', true );
}

注意事项

  • 函数自 WordPress 3.3.0 版本引入，兼容性良好
  • 用户贡献笔记中提到关于下划线前缀元键的讨论，但官方文档未明确限制，建议以实际测试为准
  • 注意参数验证：如果 $meta_type 为空或 $object_id 非数字，函数会直接返回 false
📄 原文内容

Determines if a meta field with the given key exists for the given object ID.

Parameters

$meta_typestringrequired
Type of object metadata is for. Accepts 'blog', 'post', 'comment', 'term', 'user', or any other object type with an associated meta table.
$object_idintrequired
ID of the object metadata is for.
$meta_keystringrequired
Metadata key.

Return

bool Whether a meta field with the given key exists.

Source

function metadata_exists( $meta_type, $object_id, $meta_key ) {
	if ( ! $meta_type || ! is_numeric( $object_id ) ) {
		return false;
	}

	$object_id = absint( $object_id );
	if ( ! $object_id ) {
		return false;
	}

	/** This filter is documented in wp-includes/meta.php */
	$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, true, $meta_type );
	if ( null !== $check ) {
		return (bool) $check;
	}

	$meta_cache = wp_cache_get( $object_id, $meta_type . '_meta' );

	if ( ! $meta_cache ) {
		$meta_cache = update_meta_cache( $meta_type, array( $object_id ) );
		$meta_cache = $meta_cache[ $object_id ];
	}

	if ( isset( $meta_cache[ $meta_key ] ) ) {
		return true;
	}

	return false;
}

View all references View on Trac View on GitHub

Hooks

apply_filters( “get_{$meta_type}_metadata”, mixed $value, int $object_id, string $meta_key, bool $single, string $meta_type )

Short-circuits the return value of a meta field.

Uses Description
update_meta_cache()wp-includes/meta.php

Updates the metadata cache for the specified objects.
wp_cache_get()wp-includes/cache.php

Retrieves the cache contents from the cache by key and group.
absint()wp-includes/load.php

Converts a value to non-negative integer.
apply_filters()wp-includes/plugin.php

Calls the callback functions that have been added to a filter hook.

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Used by Description
wp_save_revisioned_meta_fields()wp-includes/revision.php

Save the revisioned meta fields.
wp_install_defaults()wp-admin/includes/upgrade.php

Creates the initial content for a newly-installed site.
WP_User::__isset()wp-includes/class-wp-user.php

Magic method for checking the existence of a certain custom field.
WP_Post::__isset()wp-includes/class-wp-post.php

Isset-er.

Changelog

Version Description
3.3.0 Introduced.

User Contributed Notes

  1. Skip to note 3 content


    vutuansw


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    // Check and get a term meta

if ( metadata_exists( 'term', $term_id, '_meta_key' ) ) {
	$meta_value = get_term_meta( $term_id, '_meta_key', true );
}

// Check and get a post meta

if ( metadata_exists( 'post', $post_id, '_meta_key' ) ) {
	$meta_value = get_post_meta( $post_id, '_meta_key', true );
}

// Check and get a user meta

if ( metadata_exists( 'user', $user_id, '_meta_key' ) ) {
	$meta_value = get_user_meta( $user_id, '_meta_key', true );
}