Test if key is in user_meta array

Question

I want to associate certain users by means of a custom key stored in the user_meta for both users. There may be more than one-one relationships, so it'll be in an array. If 2 users have the same key in this _mm_group_membership array, they're part of the group.

Now since meta_value queries are restricted to simple comparisons (=, <, >, etc.) here's what I'm proposing, searching for users related to $user_id on the basis of group membership $key:

function linked_users_array ($user_id, $key) {
    $group = '_mm_group_membership';
    $users = get_users('meta_key='.$group.'&fields=all_with_meta');
    $linked_user_ids = array();
    foreach ($users as $u) {
        if (in_array($key, $u->$group)) {
            $linked_user_ids[] = $u->ID;
        }
    }
    return $linked_user_ids;
}    

2 questions on this:

1) Is $u->$group possible? Or do I need to use something along the lines of var_dump( $u->get( $group ) );?

2) I want to minimise hitting the database, so I don't really want an extra get_user_meta() call on each user in the loop. Is this possible?

Thanks in advance.

Answer 1

First of all, I believe your syntax is incorrect. Should be:

$group = '_mm_group_membership'; // key should be a string

Secondly, did you try using $u->get? Here's what the doc says:

If fields is set to all_with_meta, it will return an array of WP_User objects.

So yes, if you specified this parameter you should get a bunch of WP_User objects back. And yes, there should be a getter in the WP_User object, but you'd need to feed it that same $group variable to get the value for your key.

if($u->has_prop( $group )) {
    $meta_val = $u->get( $group );
}

Then you work with your value, which, as you mentioned, should be an array (of groups the user is in?). You might need to unserialize it, since WordPress tends to keep things as strings in the database.