How to: Change post content while publishing

Okey. So I used to have a problem with looping when I tried to change information of a post while I was publishing it.

This is my example: I wanted to post a last.fm event and the only thing you needed was to write an url in the title field and the rest of the information would be filled out.

<?php

function lastfmshows_save_show($post_id){

	if(!wp_is_post_revision($post_id)){
		global $wpdb;
		$my_post = get_post($post_id);  

		//I want to save the url in another field
		$url = $my_post->post_title;
		$split_url = explode('/' ,$url);
		$split_event = explode('+', $split_url[4]);

		//Get the data
		$fullapi = 'https://ws.audioscrobbler.com/2.0/?method=event.getinfo&format=json&event='.$split_event[0].'&api_key='. LASTFM_API;
		$xml = json_decode(file_get_contents($fullapi));

		//Only add meta if city doesn't exist
		if(get_post_meta($post_id, 'city', true)){
			return;
		}else{
			$date = DateTime::createFromFormat('D, j M Y H:i:s',$xml->event->startDate);
			add_post_meta($post_id, 'city', $xml->event->venue->location->city);
			add_post_meta($post_id, 'venue', $xml->event->venue->name);
			add_post_meta($post_id, 'url', $url);
			add_post_meta($post_id, 'id', $xml->event->id);
			add_post_meta($post_id, 'date', $date->format('ymd'));
		}
		$postdata = array(
			'ID' => $post_id,
			'post_title' => $xml->event->title,
			'post_status' => 'publish',
			'post_name' => sanitize_title($xml->event->title)
		);

		wp_update_post($postdata);

	}

}

add_action('publish_event', 'lastfmshows_save_show');

?>

At first I thought this would just create a loop since post status is “publish” and I the action trigger was on publish_event (event is my custom post type).

The thing I noticed that wasn’t working as I’m used to was add_post_meta after wp_update_post. It needs to be before. I have no idea why.

So what does this code do exactly?

1. I write an last.fm event url. Save the title to a variable so it can be used in a custom field called url.

2. I add the fields I need so they can be filled with data from last.fm.

3. Then I change the title to the new one, which is the title of the actual event from last.fm. I am also changing the permalink (post_name) to a new one matching the title.

Any thoughts? ??