Get posts by category or author

Question

I want to get posts from a particular category or if the post by a particular user . It seems we could only have AND condition in wordpress . I know the below code is wrong but this is what I need to get - I want all the posts written by a particular user OR all the posts from a particular category

$args = array(
    'posts_per_page'   => 10,
    'offset'           => $PageStart,
    'query' => array(
        'relation' => 'OR', /* <--                here */
        array(
            'author' => 18,
        ),
        array(
            'category' => 20,
        )
    ),
    'orderby' => 'date',
    'order' => 'DESC',
    'post_type' => 'post',
    'post_status' => 'publish',
);
//print_r($args);
$author_post = get_posts( $args );

Answer 1

I used the following in my child theme 'functions.php' to get posts by category name. You can also use category ID number but that didn't work for me.

add_action('pre_get_posts', 'ad_filter_categories');
function ad_filter_categories($query) {
if ($query->is_main_query() && is_home()) {
    $query->set('category_name','tips');
 }
}