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In every post of my custom post type called "announcements" I save the users' ID inside an array which resides in a meta_key called "post_is_read" whenever a user reads an announcement.

Just to explain: "post_is_read" contains an array (1, 2, 5, 12, 86, 100) where every number represents a user ID.

I achieved this using this code: https://wordpress.stackexchange.com/a/344315/15801

I am now trying to create a template which will show the posts a user has not read with the following code:

$current_user = wp_get_current_user();
$up_an_query_args = array(
    'post_type'      => 'announcement',
    'post_status'    => 'publish',
    'posts_per_page' => -1,
    'orderby'        => 'date', 
    'order'          => 'DSC',
    'meta_query' => array(
        array(
            'key'     => 'post_is_read', 
            'value'   => $current_user->ID,
            'compare' => 'NOT IN',  
        )
    ),                                      
);

I have read every related article/answer I've found both on and off wordpress stackexchange and no method worked for me. The most I could get it to work is using 'compare' => 'NOT LIKE' but it messed up the posts being displayed for users having at least a common digit in their IDs (ex between 1, 10, 11... etc) which is something I understand why is happening.

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Revised Answer

( PS: I actually wanted to revise this answer long ago, but eventually I kept forgetting to do so. :) )

So in this revised answer, the main point is the first one below, but I hope the rest also help you:

  1. What you're trying to do is not going to be (easily) possible with the way the IDs are stored, which is serialized such as a:3:{i:0;i:1;i:1;i:10;i:2;i:11;} (for array( 1, 10, 11 )).

    What if the current user ID is 10 and in the meta value, there's an i:10;i:9, i.e. array item keyed 10 with the value (a user ID) of 9 (i.e. $some_variable[10] = 9), but the ID 10 is not actually in the list?

  2. So because of the above, you might better off create a custom database table and store the user ID and post ID in their own column.

    And if you want, you can check a simplified example/demo on DB Fiddle.

  3. Or you could instead store each user ID in individual post meta named post_is_read. That way, filtering the posts would be easy — but the post meta table would end up being really huge with many, many rows...

    // Saving the meta:
    add_post_meta( 1, 'post_is_read', 10 );
    add_post_meta( 1, 'post_is_read', 11 );
    
    // Then when filtering the posts:
    $user_id = 10;
    $args = array(
        'meta_key'     => 'post_is_read',
        'meta_value'   => $user_id,
        'meta_compare' => '!=',
    );
    $query = new WP_Query( $args );
    
  4. Or you may use a serialized value, but store the user name/login and not the ID:

    // Store usernames and not IDs.
    $usernames = array( 'foo', 'user-bar', 'etc_etc' );
    
    update_post_meta( 123, 'post_is_read', $usernames );
    

    And then you can use the NOT LIKE comparison like so:

    $username = 'user-bar';
    $args = array(
        'meta_key'     => 'post_is_read',
        'meta_value'   => '"' . $username . '"', // search for exactly "<username>" (including the quotes)
        'meta_compare' => 'NOT LIKE',
    );
    $query = new WP_Query( $args );
    

Original Answer

Please check the revisions, but basically in my original answer, I was saying if you'd rather use the user IDs, then you could store them as a comma-separated list like 1,10,11 and then use the NOT REGEXP comparison:

// Saving the meta:
update_post_meta( 1, 'post_is_read', '1,10,11' );

// Then when filtering the posts:
$user_id = 10;
$args = array(
    'meta_key'     => 'post_is_read',
    'meta_value'   => "(^|,)$user_id(,|$)",
    'meta_compare' => 'NOT REGEXP',
);
$query = new WP_Query( $args );

And that did (and still does) work (with WordPress 5.6.1), except that it needs an extra parsing when retrieving the meta value, e.g.:

$ids = get_post_meta( 1, 'post_is_read', true );
$ids = wp_parse_id_list( $ids ); // convert to array
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  • Thank you for your answer Sally. Isn't it better to save the meta as serialized like the way all the data are being saved in the database by WP itself? I mean, in a sense of having a solid, well preserved/structured database. Also what should I place in my single page (or how to alter the existing code) in order to read the value in the posts loop? Is it like if ( ! in_array( $current_user->ID, $ids ) ) { ? As for the "alternate" solution, it would be a bad practice as the website I am building will have way too many users (students) and DB would end up huge indeed. – Arg Geo Aug 12 '19 at 12:50
  • WordPress serializes meta values which are array because the values have to be saved as string in the database. And I've already explained in my answer how a comma-separated list is better than a serialized string - at least in your case - but if you do not need to query the meta value using the meta_query (or meta_value and meta_compare) parameter(s), then the meta value can just be saved as a serialized string. And I didn't suggest using NOT LIKE because you've already tried it and you know it didn't work properly.. :) – Sally CJ Aug 12 '19 at 13:06
  • And regarding using it (in The Loop, etc.), basically you just need this extra part which converts the list to an array: $ids = wp_parse_id_list( $ids ); - after that, you can use in_array( $user_id, $ids ) to check if a certain user ID is within the list. – Sally CJ Aug 12 '19 at 13:09
  • Thank you very much Sally, will try your solution and let you know. I am just curious, would your method work the same way if I was storing words instead of IDs? I suppose I should use wp_parse_list instead. Is wp_parse_id_list used solely for IDs or works for any number regardless? I am sorry to bother you but I am more interested in learning how things work instead of just coping/pasting some code. – Arg Geo Aug 12 '19 at 13:36
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    It works like a charm! Thank you very much! I didn't include the "relation" though, I like the idea of setting a default meta value (0 in my case) better. – Arg Geo Aug 13 '19 at 10:04

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