meta_query only check if both value are set

Question

I have an age filter, where it checks two custom fields, min_age and max_age, max_age is optional so it can be empty.

These don't seem to work together:

    array(
      'key'     => 'max_age',
      'value'   => $max_age,
      'type' => 'numeric',
      'compare' => '<=',
    ),
    array(
      'key'     => 'max_age',
      'value'   => '',
      'compare' => '!=',
    )

If I remove the top array (<= filter), it returns the correct results, only showing results with the correct min_age and results that have a max_age value. How can I make them work together because the != '' max_age filter is needed, otherwise the max_age <= filter doesn't work correctly because it ignores that filter if the max_age custom field is empty.

      $query->set( 'meta_query', array(
        'relation' => 'AND',
        array(
          'key'     => 'min_age',
          'value'   => $min_age,
          'type' => 'numeric',
          'compare' => '>=',
        ),
        array(
          'key'     => 'max_age',
          'value'   => $max_age,
          'type' => 'numeric',
          'compare' => '<=',
        ),
        array(
          'key'     => 'max_age',
          'value'   => '',
          'compare' => '!=',
        )
      ));

Answer 1

You can use isset to check if the values are set, and then pass them as arguments. Divide your query by 2 parts and use them as needed.

// Check if both are set 
if ( isset( $max_age ) && isset( $min_age ) ) {
    $meta_query = array(
        'relation' => 'AND',
        array(
          'key'     => 'min_age',
          'value'   => $min_age,
          'type' => 'numeric',
          'compare' => '>=',
        ),
        array(
          'key'     => 'max_age',
          'value'   => $max_age,
          'type' => 'numeric',
          'compare' => '<=',
        ),
        array(
          'key'     => 'max_age',
          'value'   => '',
          'compare' => '!=',
        )
    );
} else {
    $meta_query = array(
        array(
          'key'     => 'min_age',
          'value'   => $min_age,
          'type' => 'numeric',
          'compare' => '>=',
        )
    );
}
// Now set the query
$query->set( 'meta_query', $meta_query );

If both $min_age and $max_age are always set and you want to check if they are empty or not, you can use empty() instead of isset().