我在每篇文章中使用多个位置值，因此它是一个数组（总是）。我的post meta form字段如下所示：

<select name="<?php echo $project_prefix; ?>offer_locations[]" id="<?php echo $project_prefix; ?>offer_locations" required multiple="multiple">
    ...
    ...
    ...
</select>

如果帖子已经存在，但没有任何位置值，那么现有值将出现null, 但由于该字段是必需的，[保存时]的新位置值肯定是一个数组，我们应该添加posmeta值。

我使用的条件array_diff() 工作正常。我制作了一个自定义场景并检查了代码，甚至在WordPress中，我也检查了代码(custom code block), 正如你所看到的var_dump(), echo 和exit() 内联。但是

<?php
function save_post_specifics_meta( $post_id ) {
    global $project_prefix;

    // verify nonce
    if (!isset($_POST[\'post_specifics_nonce\']) || !wp_verify_nonce($_POST[\'post_specifics_nonce\'], basename(__FILE__)))
       return $post_id;

    // check autosave
    if ( defined(\'DOING_AUTOSAVE\') && DOING_AUTOSAVE )
       return $post_id;

    // check permissions
    if ( \'post\' == $_POST[\'post_type\'] && !current_user_can(\'edit_post\', $post_id) )
    return $post_id;

    //Location meta data
    $existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $post_id );
    $new_locations      = $_POST["{$project_prefix}post_locations"]; //always an array

    var_dump($existing_locations);  //for debugging only
    var_dump($new_locations);       //for debugging only

    if( is_array( $existing_locations ) ) {
        $new_diff       = array_diff( $new_locations, $existing_locations );
        $existing_diff  = array_diff( $existing_locations, $new_locations );
    } else {
        $new_diff       = $new_locations;
        $existing_diff  = $existing_locations;
    }
    var_dump($new_diff);            //for debugging only
    var_dump($existing_diff);       //for debugging only

    if( $new_diff ) {
        foreach( $new_diff as $new_location ) {
            add_post_meta( $post_id, "{$project_prefix}post_locations", $new_location );
            echo "New Multiple: ", $new_location, \'<br>\'; //for debugging only
        }
    }

    if( $existing_locations && $existing_diff ) {
        if( is_array( $existing_diff ) ) {
            foreach ( $existing_diff as $existing_location ) {
                delete_post_meta( $post_id, "{$project_prefix}post_locations", $existing_location );
                echo "Delete Multiple: ", $existing_location, \'<br>\'; //for debugging only
            }
        } else {
            delete_post_meta( $post_id, "{$project_prefix}post_locations", $existing_diff );
            echo "Delete Single: ", $existing_diff, \'<br>\'; //for debugging only
        }
    }

    exit(); //for debugging only

}

add_action( \'save_post\',        \'save_post_specifics_meta\', 10 );
add_action( \'new_to_publish\',   \'save_post_specifics_meta\', 10 );

在保存或更新帖子时，位置元字段按预期执行。但如果数据输入操作符点击预览更改按钮，则add_post_meta() 正在触发，甚至在真正的条件括号内，并添加重复条目，I don\'t know how!!! :o

我接着补充道true 到最后一个参数（第#40行），使值唯一，以便我可以停止重复条目。但正在添加true 已停止将任何新值添加到meta_key 即使是在储蓄上。

如何停止在预览更改时添加元数据？

$existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $post_id );

if( $_POST[ \'wp-preview\' ] === \'dopreview\' ) {
    //if it\'s a preview hit, grab the actual post\'s data
    $actual_post_id     = wp_get_post_parent_id( $post_id );
    $existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $actual_post_id );
} else {
    //else, go with the actual post
    $existing_locations = get_post_custom_values( "{$project_prefix}post_locations", $post_id );
}