Comment Meta Demo Create, save and display a comment meta field. Here, a commentator can select a role.

comment-meta-demo.php

<?php # -*- coding: utf-8 -*-
namespace WPSE;
/**
 * Plugin Name: Comment Meta Demo
 * Description: Create, save and display a comment meta field. Here, a commentator can select a role.
 * Plugin URI:  http://wordpress.stackexchange.com/q/101579/73
 * Version:     2013.06.06
 * Author:      Thomas Scholz
 * Author URI:  http://toscho.de
 * Licence:     MIT
 * License URI: http://opensource.org/licenses/MIT
 */

\add_action(
	'wp_loaded',
	array( __NAMESPACE__ . '\Comment_Meta_Controller', 'init' )
);

/**
 * Controller
 *
 * Assigns Views and models to actions and filters
 */
class Comment_Meta_Controller
{
	/**
	 * Callback for add_action(). Creates a new instance.
	 *
	 * @wp-hook login_init
	 */
	public function init()
	{
		return new self;
	}

	/**
	 * Set up objects, register footer action callback.
	 *
	 * @wp-hook login_init
	 */
	protected function __construct()
	{

		$data   = new Comment_Meta_Builtin_Roles( '_comment_role' );
		// Use this for custom roles instead
		//$data   = new Comment_Meta_Custom_Roles( '_comment_role' );
		$input  = new Comment_Meta_Role_Selector( $data );
		$output = new Comment_Meta_Role_Display( $data );

		// remove this if you want to show the select field with
		// do_action( 'comment_role_selector' );
		\add_filter( 'comment_form_field_comment', array ( $input, 'show' ), 10, 2 );

		\add_action( 'comment_role_selector', array ( $input, 'print_select' ) );

		// remove this if you want to show the select field with
		// do_action( 'comment_role_selector' );
		\add_filter( 'comment_text', array ( $output, 'show' ), 10, 2 );

		\add_action( 'comment_role_value', array ( $output, 'show_action' ), 10, 2 );

		if ( 'POST' === $_SERVER[ 'REQUEST_METHOD' ] )
			\add_action( 'comment_post', array ( $data, 'save' ) );
	}
}

/**
 * Base class for handling comment meta data.
 */
abstract class Comment_Meta_Data_Model
{
	/**
	 * Meta key
	 *
	 * @type string
	 */
	protected $key;

	/**
	 * Constructor
	 *
	 * @param string $key
	 */
	public function __construct( $key )
	{
		$this->key = $key;
	}

	/**
	 * Get current key
	 *
	 * @return string
	 */
	public function get_key()
	{
		return $this->key;
	}

	/**
	 * Wrapper for the native get_comment_meta()
	 *
	 * @param  int    $comment_ID
	 * @return string
	 */
	public function get_comment_meta( $comment_ID )
	{
		$meta    = \get_comment_meta( $comment_ID, $this->key, TRUE );
		$allowed = $this->get_allowed_values();

		// get real display value
		if ( isset ( $allowed[ $meta ] ) )
			return $allowed[ $meta ];

		return '';
	}

	/**
	 * Save comment mate data.
	 *
	 * @param  int  $comment_ID
	 * @return bool
	 */
	public function save( $comment_ID )
	{
		$role_keys = array_keys( $this->get_allowed_values() );

		if ( ! isset ( $_POST[ $this->key ] ) )
			return;

		if ( ! in_array( $_POST[ $this->key ], $role_keys ) )
			return;

		return \update_comment_meta( $comment_ID, $this->key, $_POST[ $this->key ] );
	}

	/**
	 * Get user role.
	 */
	public function get_current_value()
	{
		$user = \wp_get_current_user();

		if ( empty ( $user->roles ) )
			return array ();

		return $user->roles;
	}

	/**
	 * @return array
	 */
	abstract public function get_allowed_values();
}
/**
 * User roles as comment meta.
 */
class Comment_Meta_Builtin_Roles extends Comment_Meta_Data_Model
{
	/**
	 * (non-PHPdoc)
	 * @see WPSE.Comment_Meta_Data_Model::get_allowed_values()
	 */
	public function get_allowed_values()
	{
		global $wp_roles;

		if ( empty ( $wp_roles ) )
			$wp_roles = new \WP_Roles;

		$output = array();

		foreach ( $wp_roles->roles as $identifier => $role )
			$output[ $identifier ] = $role['name'];

		return $output;
	}
}
/**
 * Custom roles for comment meta.
 */
class Comment_Meta_Custom_Roles extends Comment_Meta_Data_Model
{
	/**
	 * (non-PHPdoc)
	 * @see WPSE.Comment_Meta_Data_Model::get_allowed_values()
	 */
	public function get_allowed_values()
	{
		return array (
			'teacher' => 'Teacher',
			'student' => 'Student'
		);
	}
}

/**
 * Base class to show comment meta data.
 */
class Comment_Meta_View
{
	/**
	 * Model
	 *
	 * @type Comment_Meta_Data_Model
	 */
	protected $data;

	/**
	 * Constructor.
	 *
	 * @param Comment_Meta_Data_Model $data
	 */
	public function __construct( Comment_Meta_Data_Model $data )
	{
		$this->data = $data;
	}
}
/**
 * Show role selector from comment meta
 */
class Comment_Meta_Role_Selector extends Comment_Meta_View
{
	/**
	 * Add 'select' field before textarea.
	 *
	 * @param  string $text_field
	 * @return string
	 */
	public function show( $text_field )
	{
		return $this->get_select() . $text_field;
	}

	/**
	 * Select element.
	 *
	 * @return string
	 */
	public function get_select()
	{
		$allowed = $this->data->get_allowed_values();
		$current = $this->data->get_current_value();
		$key     = $this->data->get_key();

		$select = '<p>';
		$select .= sprintf( '<label for="%1$s_id">Your role:</label>
			<select name="%1$s" id="%1$s_id">',
			$key
		);
		$select .= '<option value="">Select a role</option>';

		foreach ( $allowed as $internal => $display )
			$select .= sprintf(
				'<option value="%1$s" %2$s>%3$s</option>',
				\esc_attr( $internal ),
				( in_array( $internal, $current ) ? 'selected' : '' ),
				\esc_html( $display )
			);

		return $select . '</select></p>';
	}

	/**
	 * Callback for do_action.
	 *
	 * @wp-hook comment_role_selector
	 * @return  void
	 */
	public function print_select()
	{
		print $this->get_select();
	}
}

/**
 * Show current comment role.
 */
class Comment_Meta_Role_Display extends Comment_Meta_View
{
	/**
	 * Add role to comment text.
	 *
	 * @wp-hook comment_text
	 * @param   string $text
	 * @param   object $comment
	 * @return  string
	 */
	public function show( $text, $comment )
	{
		$role = $this->data->get_comment_meta( $comment->comment_ID );

		if ( '' !== $role )
			$text = "Role: $role<br> $text";

		return $text;
	}

	/**
	 * Print the comment meta value into a template.
	 *
	 * Usage: <code>do_action( 'comment_role_value', 'Role: %s<br>', $comment );
	 *
	 * @wp-hook comment_role_value
	 * @param   string $template
	 * @param   object $comment
	 * @return  void
	 */
	public function show_action( $template, $comment )
	{
		$role = $this->data->get_comment_meta( $comment->comment_ID );

		if ( '' !== $role )
			printf( $template, $role );
	}
}