Extend the Jetpack Portfolio CPT to allow specifying a project URL that can be linked to instead of the portfolio item itself

class-jch-portfolio-postmeta.php

<?php

/**
 * Class JCH_Portfolio_Postmeta
 * Create meta boxes, post meta fields and archive link filters for portfolio items
 * With some help from https://gist.github.com/carlodaniele/121841f92956c3304436
 */


class JCH_Portfolio_Postmeta {

	public function __construct() {
		add_action( 'init', array( $this, 'init' ), 1 );
	}

	public function init() {
		add_action( 'add_meta_boxes_jetpack-portfolio', array( $this, 'portfolio_add_meta_boxes' ) );
		add_action( 'save_post_jetpack-portfolio', array( $this, 'portfolio_save_meta_box_data' ) );
		add_filter( 'post_type_link', array( $this, 'portfolio_permalink' ) );
	}

	/**
	 * Add meta box
	 *
	 * @param post $post The post object
	 * @link https://codex.wordpress.org/Plugin_API/Action_Reference/add_meta_boxes
	 */
	public function portfolio_add_meta_boxes( $post ) {
		add_meta_box(
			'portfolio_meta_box',
			'Project URL',
			array( $this, 'portfolio_build_meta_box' ),
			'jetpack-portfolio',
			'normal',
			'high'
		);
	}

	/**
	 * Build custom field meta box
	 *
	 * @param post $post The post object
	 */
	public function portfolio_build_meta_box( $post ) {

		// make sure the form request comes from WordPress
		wp_nonce_field( basename( __FILE__ ), 'portfolio_meta_box_nonce' );

		// retrieve the _project_url current value
		$current_project_url = get_post_meta( $post->ID, '_project_url', true );

		?>
		<div class='inside'>
			<p>If this field is populated with a URL, the archive display of the item will link directly to the URL.</p>
			<p>
				<input type="text" size="60" name="project_url" title="Project URL" value="<?php echo esc_url( $current_project_url ); ?>"/>
			</p>

		</div>
		<?php
	}

	/**
	 * Store custom field meta box data
	 *
	 * @param int $post_id The post ID.
	 * @link https://codex.wordpress.org/Plugin_API/Action_Reference/save_post
	 */
	public function portfolio_save_meta_box_data( $post_id ) {
		// verify meta box nonce
		if ( ! isset( $_POST['portfolio_meta_box_nonce'] ) || ! wp_verify_nonce( $_POST['portfolio_meta_box_nonce'], basename( __FILE__ ) ) ) {
			return;
		}

		// return if autosave
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return;
		}

		// Check the user's permissions.
		if ( ! current_user_can( 'edit_post', $post_id ) ) {
			return;
		}

		// store custom fields values
		// project_url
		if ( isset( $_REQUEST['project_url'] ) ) {
			update_post_meta( $post_id, '_project_url', sanitize_text_field( $_POST['project_url'] ) );
		}
	}

	/**
	 * For portfolio items, filter the permalink to go directly to the project URL if one exists
	 * @param $permalink
	 * @return string
	 */
	public function portfolio_permalink( $permalink ) {

		if ( 'jetpack-portfolio' === get_post_type() ) {
			$project_url = get_post_meta( get_the_ID(), '_project_url', true );
			if ( ! empty( $project_url ) && wp_http_validate_url( $project_url ) ) {
				return esc_url( $project_url );
			}
		}

		return $permalink;
	}
}

$jch_portfolio_postmeta = new JCH_Portfolio_Postmeta();