Example for add a Custom Post Type to the WordPress Loop

gistfile1.txt

// add custom post type to wp loop
add_action( 'pre_get_posts', 'add_to_query' );

// ads to query
function add_to_query( $query ) {
	
	if ( is_admin() || is_preview() )
		return;

	// only filter on front page
	if ( is_home() || is_front_page() && ( FALSE == $query -> query_vars['suppress_filters'] ) ) {
		$query->set( 'post_type', array( 'post', 'my_post_type' ) );
	
	return $query;
}

I think it is better you use is_singular() for posts and pages or use the line 11 for only add the filter on a specific sceanrio - id ( ! is_singular() && ( FALSE == $query -> query_vars['suppress_filters'] ) ) .

Hook int correcty is an action --> add_action(); not add_filter() - Sorry; its fixed on example

Thanks!