我使用WordPress Meta Box Generator v2 Beta网站生成了一个自定义的Meta Box，将自定义图像文件上传到自定义帖子中，在管理端，一切似乎都很好，但我不知道如何在前端进行输出。所以我尝试了<?php echo get_post_meta( get_the_id(), \'project_logo_logo-image\', true ); ?> 但似乎什么都不起作用，也没有任何产出。请你给我指一下正确的方向好吗？

    class Rational_Meta_Box {
    private $screens = array(
        \'portfolio_post_type\',
    );
    private $fields = array(
        array(
            \'id\' => \'logo-image\',
            \'label\' => \'Logo Image\',
            \'type\' => \'media\',
        ),
    );

    /**
     * Class construct method. Adds actions to their respective WordPress hooks.
     */
    public function __construct() {
        add_action( \'add_meta_boxes\', array( $this, \'add_meta_boxes\' ) );
        add_action( \'admin_footer\', array( $this, \'admin_footer\' ) );
        add_action( \'save_post\', array( $this, \'save_post\' ) );
    }

    /**
     * Hooks into WordPress\' add_meta_boxes function.
     * Goes through screens (post types) and adds the meta box.
     */
    public function add_meta_boxes() {
        foreach ( $this->screens as $screen ) {
            add_meta_box(
                \'project-logo\',
                __( \'Project Logo\', \'yopta\' ),
                array( $this, \'add_meta_box_callback\' ),
                $screen,
                \'normal\',
                \'default\'
            );
        }
    }

    /**
     * Generates the HTML for the meta box
     * 
     * @param object $post WordPress post object
     */
    public function add_meta_box_callback( $post ) {
        wp_nonce_field( \'project_logo_data\', \'project_logo_nonce\' );
        echo \'Add Project Logo Image\';
        $this->generate_fields( $post );
    }

    /**
     * Hooks into WordPress\' admin_footer function.
     * Adds scripts for media uploader.
     */
    public function admin_footer() {
        ?><script>
            // https://codestag.com/how-to-use-wordpress-3-5-media-uploader-in-theme-options/
            jQuery(document).ready(function($){
                if ( typeof wp.media !== \'undefined\' ) {
                    var _custom_media = true,
                    _orig_send_attachment = wp.media.editor.send.attachment;
                    $(\'.rational-metabox-media\').click(function(e) {
                        var send_attachment_bkp = wp.media.editor.send.attachment;
                        var button = $(this);
                        var id = button.attr(\'id\').replace(\'_button\', \'\');
                        _custom_media = true;
                            wp.media.editor.send.attachment = function(props, attachment){
                            if ( _custom_media ) {
                                $("#"+id).val(attachment.url);
                            } else {
                                return _orig_send_attachment.apply( this, [props, attachment] );
                            };
                        }
                        wp.media.editor.open(button);
                        return false;
                    });
                    $(\'.add_media\').on(\'click\', function(){
                        _custom_media = false;
                    });
                }
            });
        </script><?php
    }

    /**
     * Generates the field\'s HTML for the meta box.
     */
    public function generate_fields( $post ) {
        $output = \'\';
        foreach ( $this->fields as $field ) {
            $label = \'<label for="\' . $field[\'id\'] . \'">\' . $field[\'label\'] . \'</label>\';
            $db_value = get_post_meta( $post->ID, \'project_logo_\' . $field[\'id\'], true );
            switch ( $field[\'type\'] ) {
                case \'media\':
                    $input = sprintf(
                        \'<input class="regular-text" id="%s" name="%s" type="text" value="%s"> <input class="button rational-metabox-media" id="%s_button" name="%s_button" type="button" value="Upload" />\',
                        $field[\'id\'],
                        $field[\'id\'],
                        $db_value,
                        $field[\'id\'],
                        $field[\'id\']
                    );
                    break;
                default:
                    $input = sprintf(
                        \'<input %s id="%s" name="%s" type="%s" value="%s">\',
                        $field[\'type\'] !== \'color\' ? \'class="regular-text"\' : \'\',
                        $field[\'id\'],
                        $field[\'id\'],
                        $field[\'type\'],
                        $db_value
                    );
            }
            $output .= $this->row_format( $label, $input );
        }
        echo \'<table class="form-table"><tbody>\' . $output . \'</tbody></table>\';
    }

    /**
     * Generates the HTML for table rows.
     */
    public function row_format( $label, $input ) {
        return sprintf(
            \'<tr><th scope="row">%s</th><td>%s</td></tr>\',
            $label,
            $input
        );
    }
    /**
     * Hooks into WordPress\' save_post function
     */
    public function save_post( $post_id ) {
        if ( ! isset( $_POST[\'project_logo_nonce\'] ) )
            return $post_id;

        $nonce = $_POST[\'project_logo_nonce\'];
        if ( !wp_verify_nonce( $nonce, \'project_logo_data\' ) )
            return $post_id;

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

        foreach ( $this->fields as $field ) {
            if ( isset( $_POST[ $field[\'id\'] ] ) ) {
                switch ( $field[\'type\'] ) {
                    case \'email\':
                        $_POST[ $field[\'id\'] ] = sanitize_email( $_POST[ $field[\'id\'] ] );
                        break;
                    case \'text\':
                        $_POST[ $field[\'id\'] ] = sanitize_text_field( $_POST[ $field[\'id\'] ] );
                        break;
                }
                update_post_meta( $post_id, \'project_logo_\' . $field[\'id\'], $_POST[ $field[\'id\'] ] );
            } else if ( $field[\'type\'] === \'checkbox\' ) {
                update_post_meta( $post_id, \'project_logo_\' . $field[\'id\'], \'0\' );
            }
        }
    }
}
new Rational_Meta_Box;