Updating meta_value adding an associative array to those that already exist

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What I’m trying to do is to optimize the way to store data in my wp_postmeta table.

I need to save multiple relations between posts, and my starting situation was simply this:

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add_post_meta($parent_id, 'post_child', $child_id);

In that way I needed to use a database row for each relation.

Considering that the same parent can be associated to multiple children, I was trying to figure it out what could be a good array configuration, and I got to something like this (but I’m still not sure is the best way): 

array(
    array(
        parent => 121,
        child => 122
    ),
    array(
        parent => 121,
        child => 122
    ),
    array(
        parent => 121,
        child => 123
    ),
    ...
);

Then, I tried with this code: 

if ($post_relations = get_post_meta($book_id, 'post_relations', true)) {

    $post_relations[] = array("parent" => $parent_id, "child" => $child_id);

    update_post_meta($book_id, 'post_relations', $post_relations);

} else {

    $post_relations[] = array("parent" => $parent_id, "child" => $child_id);

    add_post_meta($book_id, 'post_relations', $post_relations);

}

But the result I get in the meta_value field seems to be different to the result I was expecting: 

a:2:{
i:0;a:2:{s:6:"parent";i:1;s:5:"child";i:510;}i:1;a:2:{s:6:"parent";i:510;s:5:"child";i:511;}
}
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2 comments

  1. The WordPress functions update_post_meta() and add_post_meta() both require serializable data;

    update_post_meta ( int $post_id, string $meta_key, mixed $meta_value, mixed $prev_value = '' )

    $meta_valueMust be serializable if non-scalar

    update_post_meta()

    It looks like this in the database because those functions serialize() the array you’re passing it.

    Therefore you if you were accessing the data outside of the get_post_meta() function you would have to unserialize() the data.

  2. I think you should save the relations as one relation per one post meta – saving the relations in the array would make DB queries very difficult.

    So, either save your relations as ie post_relations_0, post_relations_1 etc, or, even better, if you plan to do some queries, save the relations in separate table.