Appending meta value onto post content in WordPress during save_post

Question

The problem seems pretty straight forward: Once the save_post action is fired, I would like to append the value of a meta key onto the end of the post_content if it's not already found within the post itself.

I've attempted making a call to wp_update_post() from within a function tied to save_post, but as many of you will immediately object, wp_update_post() contains do_action('save_post') and as such creates an infinite loop.

What other way (using the API) exists for one to update the content of a post based on said posts custom values once the post has been saved/edited without invoking a nightmarish infinite loop?

Answer 1

The initial solution I've found is to tie a filter onto an action like wp_insert_post_data and extract other post information from the global $post object.

// Tack our filter onto the wp_insert_post_data action
add_filter( 'wp_insert_post_data', 'my_appender' );
function my_appender( $content ) {
  // Bring global $post into scope
  global $post;
  // Get meta value of meta key 'key_name'
  $meta_value = get_post_meta( $post->ID, 'key_name', TRUE );
  // If value is not in content, append it onto the end
  if ( stristr( $content['post_content'], $meta_value ) === FALSE )
    $content['post_content'] .= $meta_value;
  // Return filtered content
  return $content;
}

I'm certain this could see improvement.

References

1. add_filter() - "Filters are the hooks that WordPress launches..."
2. wp_insert_post_data - "A filter hook called by the wp_insert_post function..."
3. global keyword - "The scope of a variable is the context within which it is defined..."
4. get_post_meta() - "This function returns the values of the custom fields..."
5. stristr() - "Find first occurrence of a string (Case-insensitive)..."