I have a two custom taxonomies:

• Type
• Product

Type can be the following:

1. Blog
2. Case study
3. Webinar

Subject can be:

1. Indoor
2. Outdoor

I have two drop down menus in WordPress backend where a user can select what type and subject they want to show from resources (my custom post type).

Say, for example, the following posts exist in resources:

• Post 1: Tagged with type blog and subject outdoor.
• Post 2: Tagged with type blog and subject indoor.
• Post 3: Tagged with subject indoor.

The user can either filter by type or subject. By this I mean not both are required, only one can be selected. But, if the user does choose both a type and subject, I want it to display posts with both tags.

Current approach:

$args = array(  
  'post_type' => 'resources',
  'post_status' => 'publish',
  'posts_per_page' => 8,
  'tax_query' => array(
    'relation' => 'AND',
    array(
        'taxonomy' => 'type',
        'field'    => 'slug',
        'terms'    => $resource_type,
    ),
    array(
        'taxonomy' => 'subject',
        'field'    => 'slug',
        'terms'    => $resource_subject,
    ),
  ),
  'tax_query' => array(
    'relation' => 'OR',
    array(
        'taxonomy' => 'type',
        'field'    => 'slug',
        'terms'    => $resource_type,
    ),
    array(
        'taxonomy' => 'subject',
        'field'    => 'slug',
        'terms'    => $resource_subject,
    ),
  )

);

• $resource_type: Is the variable holding the dropdown value for
  type.
• $resource_subject: Is the variable holding the dropdown
  value for subject.

Current results:

• When filtering by subject alone – it works.

• When filtering by type alone – it works.

• When filtering with both – it doesn’t work. I.e. I’ve filtered by type blog and subject indoor and it is showing me subject outdoor posts.

Alternatively, the following works when querying both, but does not work when only choosing one:

 $args = array(  
    'post_type' => 'resources',
    'post_status' => 'publish',
    'posts_per_page' => $card_count,
    'tax_query' => array(
      'relation' => 'AND',
      array(
          'taxonomy' => 'type',
          'field'    => 'slug',
          'terms'    => $resource_type,
      ),
      array(
          'taxonomy' => 'subject',
          'field'    => 'slug',
          'terms'    => $resource_subject,
      ),
    )

  );