Include post id[s] into WP_Query()

Question

I'm assigning some posts to some users whom are not the author of those posts. Typically private posts are visible only to the post authors. So, if the author has 3 posts, on his logged in edit.php they will get 3 posts. But if another post is assigned to them by custom field (not as a post author), how can I add the post to the post list table?

What I tried so far is: making a function (custom_get_assigned_posts()) to retrieve all the posts that are assigned to a user_id as an array. But then found that there's no easy way including post IDs into the existing query:

function add_assigned_posts( $query ) {
    if( is_admin() && in_array( $query->get('post_type'), array('mycpt') ) ) {
        global $current_user;
        $query->set('post__in', custom_get_assigned_posts( $current_user->ID ) );
    }
    return $query;
}
add_filter( 'pre_get_posts', 'add_assigned_posts' );

But post__in doesn't additionally include things, it actually specified posts to retrieve.

How can I include posts that are assigned to a non-author of posts, and are assigned by postmeta?

Answer 1

I think you should be able to do this with the user_has_cap filter.

Because that filter passes through the post ID in the $args for the edit_post capability, you can use that to check if the current user has been artificially given this capability - and if so, force the edit_post capability to be true for that instance.

Building from the example in the docs linked above, you'd be looking at something like this:

add_filter( 'user_has_cap', 'wpse_227418_edit_extra_posts', 10, 3 );

function wpse_227418_edit_extra_posts($allcaps, $cap, $args){

  if($args[0] != "edit_post"){
    return $allcaps; // bail out, we're not talking about editing a post here
  }

  if(get_post_meta($args[2], "additional_author", true) == $args[1]){
    // get the meta key 'additional author', checking that it matches the User ID we're looking at the capabilities for
    $allcaps[$cap[0]] = true; // make sure edit_posts is true for this author and this post ID
  }

  return $allcaps;

}

This is pretty quickly put together and is untested so you'd certainly want to test and modify to your needs, but I think this will get you what you want. Of course, adjust the meta_key referred to above to match the key you're after.

References:

• user_has_cap filter on Codex
• has_cap() function in WP_User class (the function that calls the above filter)