Registering dependency on 'wp-posts-to-posts' from a custom plugin

Question

I have custom plugin which registers multiple CPT's and i want to reuse the features of wp-posts-to-posts to define relationships. I'm just wondering what is the correct/best way to include the api.php file within my own plugin?

function my_connection_types() {

    require_once( ABSPATH . 'wp-content/plugins/posts-to-posts/core/api.php' );
    // Make sure the Posts 2 Posts plugin is active.
    if ( !function_exists( 'p2p_register_connection_type' ) )
        return;

    p2p_register_connection_type( array(
            'name' => 'events_to_races',
            'from' => 'event',
            'to' => 'race',
            'cardinality' => 'one-to-many'
    ) );
}

Answer 1

Don't try to load the plugin yourself, check for its existence on your activation hook.

If it's not activated, throw an error; WordPress will display the message & prevent activation of your plugin.