WordPress – Filtering Custom Post Type by multiple Custom Taxonomies on front end

I’m trying to allow users to select criteria from multiple custom taxonomies in order to filter a custom post type on the front end via a from with multiple selects.

My problem is that the code only works if each of the selects has criteria selected. Trying to find a way where users can also just select one of the options and it will only filter the CPT by that option.

* All code is abstracted. Taxonomy names are actually: incident_category, incident_classification, incident_type, et al.

Markup:

<form action="<?php echo site_url() ?>/wp-admin/admin-ajax.php" method="POST" id="filter">
    <?php
        if( $terms = get_terms( array(
            'taxonomy' => 'tax1',
        ) ) ) :
            echo '<select name="tax1filter"><option value="">Select tax1...</option>';
            foreach ( $terms as $term ) :
                echo '<option value="' . $term->term_id . '">' . $term->name . '</option>';
            endforeach;
            echo '</select>';
        endif;

        if( $terms = get_terms( array(
            'taxonomy' => 'tax2',
        ) ) ) :
            echo '<select name="tax2filter"><option value="">Select tax2...</option>';
            foreach ( $terms as $term ) :
                echo '<option value="' . $term->term_id . '">' . $term->name . '</option>';
            endforeach;
            echo '</select>';
        endif;

        if( $terms = get_terms( array(
            'taxonomy' => 'tax3',
        ) ) ) :
            echo '<select name="tax3filter"><option value="">Select tax3...</option>';
            foreach ( $terms as $term ) :
                echo '<option value="' . $term->term_id . '">' . $term->name . '</option>';
            endforeach;
            echo '</select>';
        endif;
    ?>
    <button>Apply filter</button>
    <input type="hidden" name="action" value="myfilter">
</form>
<div id="loop"></div>

JS:

jQuery(function($){
    $('#filter').submit(function(){
        var filter = $('#filter');
        $.ajax({
            url:filter.attr('action'),
            data:filter.serialize(), // form data
            type:filter.attr('method'), // POST
            beforeSend:function(xhr){
                filter.find('button').text('Processing...'); // changing the button label
            },
            success:function(data){
                filter.find('button').text('Apply filter'); // changing the button label back
                $('#loop').html(data); // insert data
            }
        });
        return false;
    });
});

Function:

add_action('wp_ajax_myfilter', 'misha_filter_function'); // wp_ajax_{ACTION HERE}
add_action('wp_ajax_nopriv_myfilter', 'misha_filter_function');

function misha_filter_function(){
    $args = array(
        'post_type' => 'cpt',
        'orderby' => 'date',
        'order'    => 'DESC',
    );

    $relation = 'OR';

    if(isset($_POST['tax1filter']) && isset( $_POST['tax2filter'] ) && isset( $_POST['tax3filter'] )) {
        $relation = 'AND';
    }
    if( isset( $_POST['tax1filter'] ) ||  isset( $_POST['tax2filter'] || isset( $_POST['tax3filter']) )
        $args['tax_query'] = array(
            'relation' => $relation, 
            array(
                'taxonomy' => 'tax1',
                'field' => 'id',
                'terms' => $_POST['tax1filter']
            ),
            array(
                'taxonomy' => 'tax2',
                'field' => 'id',
                'terms' => $_POST['tax2filter'],
            ),
            array(
                'taxonomy' => 'tax3',
                'field' => 'id',
                'terms' => $_POST['tax3filter'],
            ),
        );


    $query = new WP_Query( $args );

    if( $query->have_posts() ) :
        while( $query->have_posts() ): $query->the_post();
            echo '<h5>' . $query->post->post_title . '</h5>';
        endwhile;
        wp_reset_postdata();
    else :
        echo 'No posts found';
    endif;

    die();
};

Any help with this would be as always greatly appreciated!

Answer

This seems to get what you need:

function misha_filter_function(){
    $taxonomies = get_taxonomies();;
    $args = array(
        'post_type' => 'cpt',
        'orderby' => 'date',
        'order'    => 'DESC',
    );
    $relation = 'AND';
    $params = array();
    $args['tax_query']['relation'] = $relation;

    foreach ( $taxonomies as $tax ) {
        if( isset( $_POST[ $tax . 'filter' ] ) && !empty( $_POST[ $tax . '_filter' ] ) {
            $args['tax_query'][] = array(
                    'taxonomy' => $tax,
                    'field' => 'id',
                    'terms' => $_POST[ $tax . 'filter' ],
                );
        }
    }

    $query = new WP_Query( $args );

    if( $query->have_posts() ) :
        while( $query->have_posts() ): $query->the_post();
            echo '<h5>' . $query->post->post_title . '</h5>';
        endwhile;
        wp_reset_postdata();
    else :
        echo 'No posts found';
    endif;

    die();
};