wp_insert_post_data filter to set category

Question

I am trying to set additional "default" categories. I figured by hooking into the wp_insert_post_data filter, I could use $postarr to set the category. It doesn't seem to be working? Can someone offer advice as to how I would go about doing this?

add_filter( 'wp_insert_post_data', 's3x9s_wp_insert_post_data', '99', 2 );
function s3x9s_wp_insert_post_data( $data, $postarr ) {
    array_push( $postarr['post_category'], 123 );
    return $data;
}   

Answer 1

I think that the best way to set the default category is in the wp-admin: Settings > Writings > Default post Category However, I think you're looking for setting more than one category by default.

If you look the documentation of wp_insert_post_data, there isn't any key such as post_category.

Perhaps, you should use publish_post

Runs when a post is published, or if it is edited and its status is "published". Action function arguments: post ID.

There, you should use wp_set_post_categories

So, the code would be something similar to this:

function set_category_by_default( $post_ID ) {
    wp_set_post_categories( $post_ID, array(123, 124) ) ;
}
add_action( 'publish_post', 'set_category_by_default', 5, 1 );