My combination of 'post_type' and 'tax_query' not working?

Question

I want to filter search results by custom post type and custom taxonomy terms, the result is here:

add_action('pre_get_posts','search_filter');
function search_filter($query) {
    if($query->query_vars['s'] != '' && is_search())
    {
        $query->set('post_type', array('partner','event','product'));
        $tax_query = array(
            'relation' => 'OR',
            array(
                'taxonomy' => 'city',
                'field'    => 'slug',
                'terms'    => array( 'belgrade' )
            )
        );
        $query->set( 'tax_query', $tax_query );
    }
}

Posts from specific CPT works, but filter by a category, not. Why? :)

Answer 1

I think the issue here is you're using the relation where you only have one inner tax array. So, it should be without one.

$tax_query = array(
      array(
         'taxonomy' => 'city',
         'field'    => 'slug',
         'terms'    => array( 'belgrade' )
   )
);

Here's a detailed explanation from WP: Taxonomy Parameters

• relation (string) – The logical relationship between each inner taxonomy array when there is more than one. Possible values are ‘AND’, ‘OR’. Do not use with a single inner taxonomy array.