Tax query with multiple terms in pre_get_posts

Question

I am trying to use a tax query in my pre_get_posts function. All is working but not when the array has multiple IDs. Let's say my URL looks like: ?listing_cat[]=1&listing_cat[]=2

Now I am trying to filter out posts that are in those 2 categories. But my filter only gets the posts from the first category ID. I guess I will need a foreach function, but I am not sure how to implement this. Currently I have the code below. Now I know I can use filter by multiple terms with a comma, like : 'terms' => array($rt_cat_id[0], $rt_cat_id[1]), but I just need it to automatically get all the terms from the array $rt_cat_id and use those to filter the posts. How to accomplish that? Thanks

add_action( 'pre_get_posts', 'rt_tax_archive' );
function rt_tax_archive($query) {
$rt_cat_id = $_GET['listing_cat'];

if( isset( $rt_cat_id ) && ! empty( $rt_cat_id[0] ) ) {
            $tax_query[] = array(
                'taxonomy'  => 'listing_category',
                'field'     => 'id',
                'terms'     => array($rt_cat_id[0]),
           );
        }       


}

Answer 1

Just use:

'terms' => $rt_cat_id

I'll work for both array and single-based values.

Or you can simplify your code as follows:

if( isset( $_GET[ 'listing_cat' ] ) ) {
   $tax_query[] = array(
      'taxonomy' => 'listing_category',
      'field' => 'id',
      'terms' => $_GET[ 'listing_cat' ]
   );
}

Answer 2

So, the working code can be found below:

add_action( 'pre_get_posts', 'rt_tax_archive' );
function rt_tax_archive($query) {
$rt_cat_id = $_GET['listing_cat'];

if( isset( $rt_cat_id ) && ! empty( $rt_cat_id[0] ) ) {
            $tax_query[] = array(
            'relation' => 'OR',
            array(
            'taxonomy' => 'listing_category',
            'field' => 'id',
            'terms' => $rt_cat_id,
            ),
        );
        }       


}