Add Custom Post Type to Current Query

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You are actually not that far away with pre_get_posts, you just need to first grab the current post types that are in the query before adding your own.

Also, I’d recommend using a $priority of 99 so that the action is hooked after any other pre_get_posts hooks, meaning that if users add there own CPT’s (or built in post types) they are detected and included by the function below (but you can change that as required).

add_action('pre_get_posts', 'djg_includ_my_cpt_in_query', 99);
function djg_includ_my_cpt_in_query($query){

    if(is_home() && $query->is_main_query()) :              // Ensure you only alter your desired query

        $post_types = $query->get('post_type');             // Get the currnet post types in the query

        if(!is_array($post_types) && !empty($post_types))   // Check that the current posts types are stored as an array
            $post_types = explode(',', $post_types);

        if(empty($post_types))                              // If there are no post types defined, be sure to include posts so that they are not ignored
            $post_types[] = 'post';         
        $post_types[] = 'document';                         // Add your custom post type

        $post_types = array_map('trim', $post_types);       // Trim every element, just in case
        $post_types = array_filter($post_types);            // Remove any empty elements, just in case

        $query->set('post_type', $post_types);              // Add the updated list of post types to your query

    endif; 

    return $query;

}

EDIT

When constructing the final query, WordPress checks to see if ‘post_type’ is empty, and if so the following code is executed –

$where .= " AND $wpdb->posts.post_type="post"";
$post_type_object = get_post_type_object ( 'post' );

So what happens when we add a CPT is that WordPress assuems we want to ignore posts as it is not explicitly declared. That is were my scenario differed – I had explicitly declared other post types previously so they were included in the call $post_types = $query->get('post_type');.

So, in your case we can resonably summise that if $post_types is empty, the user does not wish to amend that portion of the query so ‘post’ can be manually added to the $post_types array. This should (hopefully!) do the trick.

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