Custom Fields in your Feed

Since I work very often with custom fields and integrate various functions in my theme, I noticed something which I use extensively. I fill my feed exclusively with content from the custom fields.
For example, a short additional text (thumbnails, information, links, etc.):

function feed_monkey( $content ) {
    global $post, $id;

    if ( !is_feed() )
        return $content;

    $attachment = get_post_meta( $post->ID, 'Attachment', $single = true );

    // Print custom fields ***
    if ( $attachment != '' )
        return __('more information: ') . $attachment;
}

add_filter( 'the_content', 'feed_monkey' );

It works for me with a whole series of custom fields. With this solution, only the content of the custom fields are included in the feed, not the content of the actual content, very useful for photoblogs.

To attach the content of the custom fields to the actual content, you must output the variable $content:

    // Print custom fields and Content
    if ( $attachment != '' )
        return $content . '
' . __('Additionalinformation: ') . $attachment;