Searching using meta_query in wordpress - php

Question

I'm using wordpress to search some content with a few restrictions. Every time I save a post, I save in meta a related posts array.
Then I'm trying to search all posts which has a certain post as related, I have the following query. Selected post is an id to the post I'm looking for.
I checked $selected_post and it has values like the following one.
$selected_post = "25";
I added the meta value using the following instruction, I'm using update_post_meta. $related_posts contains an array of post ids, like the example below.
update_post_meta( $post->ID, 'related_post', $related_posts );
Query
$arg = array(
'post_type' => 'post',
'posts_per_page' => 5,
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'related_post',
'value' => array( $selected_post ),
'compare' => 'IN',
'type' => 'STRING'
),
),
);
I checked this meta in some of my current posts, and I have something like this.
'related_post' => array( '15', '25', '46' );
When I execute this query using WP_Query it always returns me an empty array. I think I need something more in the query to make this works.
Any help would be appreciated.

Answer 1

The problem is that you are trying to query related_posts as if it was an array, but it actually gets serialised when its added to the database.
Using WP_Query on serialized arrays
If you were to add an array of post meta as follows:
$fruit_array = array( 'apple', 'orange', 'banana' );
update_post_meta( $post->ID, 'fruit', $fruit_array );
...the value for fruit in the database would be:
a:3:{i:0;s:5:"apple";i:1;s:6:"orange";i:2;s:6:"banana";}
Therefore you need to use LIKE to search the serialised string for the value you are looking for.
For the above example, your $args would be something like:
$args = array(
'post_type' => 'post',
'posts_per_page' => 5,
'meta_query' => array(
array(
'key' => 'fruit',
'value' => 'apple',
'compare' => 'LIKE',
),
)
);
Search for numbers/post ids
In your case, this is complicated by the fact that you are using post ids. Post ids or numbers aren't an issue specifically, but the problem is that LIKE will also return partial matches, so a query for 14 would also return 141, 1455 etc.
Based on the structure of the serialised string, I believe the following should work:
$args = array(
'post_type' => 'post',
'posts_per_page' => 5,
'meta_query' => array(
array(
'key' => 'related_post',
'value' => '"25"', /* include the double quotes in the search value */
'compare' => 'LIKE',
),
)
);
As the values in the serialised string becomes "25"; including the double quotes in the value you want to search for should work.