Hi @Sara!

Happy to help you with this. 🙂

First, did you happen to see this in the documentation? https://piklist.github.io/docs/tutorials/relationships/working-with-field-relationships/

Let’s break down your first query:
'field' => 'city_event',
You shouldn’t need this. If you do need it for some reason then the relate field isn’t working.

  'choices' => piklist(get_posts(array(
    'numberposts' => -1,
    'post_type' => 'city',
  )),
  array('ID', 'post_title')),

What’s happening there is that get_posts() is retrieving an array of WP_Post objects that contains all of your city posts. From there, the piklist() function is “plucking” the array, meaning that it’s creating a new array from the WP_Post array wherein the new array key is the ID and the value is the post_title. For a relate field to work it needs the value stored (key) to be the object ID, and the post_title is so the user knows what they’re looking at when selecting an option.

'relate' => array( 'scope' => 'post' ),

This is the “magic” line that sets the field up as a relate field. The scope here would either be ‘post’, ‘user’, or ‘comment’, based on what you’re connecting to. Since you’re connecting two posts, the scope would be ‘post’. If you see 'scope' in the context of the root field parameters, that’s for non-relate fields and most times doesn’t need to be worried about.

Here’s a working example of how I would structure what you’re doing:

piklist('field', array(
  'type'      => 'select',
  'label'     => 'Accociated City'
  'choices'   => piklist(get_posts(array(
    'post_type'   => 'city',
    'numberposts' => -1
  )), array('ID', 'post_title')),
  'relate'    => array(
    'scope'     => 'post'
  ),
  'attributes'=> array(
    'multiple'  => 'multiple',
  )
));

—

$related_events = get_posts(array(
    'post_type' => 'event',
    'numberposts' => -1,
    'post_belongs' => $post->ID,
       'suppress_filters' => false,
       'relate_query' => array(
          array(
            'id' => $post->ID,
            'relate' => 'belongs'
           ))
     ));

For this, you did get just a bit mixed up. Here’s what it should be structured like:

$related_events = get_posts(array(
  'post_type'        => 'event',
  'numberposts'      => -1
  'suppress_filters' => false,
  'relate_query'     => array(
    array(
      'id'    => $post->ID,
      'relate'=> 'has'
    )
  )
));

First, I put 'suppress_filters' in the root level parameters where it belongs.

Next, I changed 'relate' to 'has'. The reason is that since the city selection is on the event page, the event has the cities, not the other way around. And since we’re querying an event, we’re saying we want to limit the events by which cities the queried event has.

Hope this helps! I think the example in the docs should help, too!

Glad that was helpful! 🙂

Let’s talk about how relate data is stored for a moment. In your case you have an event post type relating to a city post type. As mentioned, in this case the event owns the city; that is to say, when creating an event the user selects which city it will be in — makes sense!

The way Piklist stores this is that in the city’s meta it adds a __relate_post meta key wherein the value is the ID of the event that owns it. This is later used when performing relate queries.

So if you look at the event’s meta, you won’t see anything added about the city. Why? Because it’s not designed to put anything there. Look at the city’s meta. You should find it there. For this reason, you don’t need to specify a field, as relate fields are a special type which store in their own way.

And, yes, if you only want one then remove the multiple line.

—

To debug this, first check the city’s meta to see if the relate data is being stored.

Next, double-check your query. Is $post a WP_Post object as you’re expecting? Is the ID what you’re expecting? Make sure you have 'suppress_filters' => false, otherwise WordPress will restrict WordPress from adjusting the query for the relate parameters (annoying that true is the WP default for that function, but is false for WP_Query queries).

If that doesn’t work, zip up the related files and shoot them over to [email protected] so we can take a closer look. I’m quite sure it’s something simple, though, and if you can figure it out you’ll be the better for it!

—

Quick tip:
Since you’re doing a simple query, you can do a shorthand relate format like this:

$related_events = get_posts(array(
  'post_type'        => 'event',
  'numberposts'      => -1
  'suppress_filters' => false,
  'post_has'         => $post->ID
));

Much easier to read and is great for simple queries. 🙂