Modified mp_related_products function (v2)

<?php

if (!function_exists('mp_related_products')) :
/**
 * Displays related products for the passed product id
 *
 * @param int $product_id.
 * @param bool $in_same_category Optional, whether to limit related to the same category.
 * @param bool $echo. Optional, whether to echo or return the results
 * @param int $limit. Optional The number of products we want to retrieve.
 * @param bool $simple_list Optional, whether to show the related products based on the "list_view" setting or as a simple unordered list
 * @param bool $in_same_tags Optional, whether to limit related to same tags
 */
function mp_related_products() {
        global $mp, $post;

        $output = '';
        $categories = $tag_list = array();

        if( $mp->get_setting('related_products->show') == 0)
                return '';

        $defaults = array_merge($mp->defaults['related_products'], array(
                'simple_list' => $mp->get_setting('related_products->simple_list'),
                'relate_by' => $mp->get_setting('related_products->relate_by'),
                'limit' => $mp->get_setting('related_products->show_limit'),
        ));

        $args = $mp->parse_args(func_get_args(), $defaults);

        if( !is_null($args['product_id']) ) {
                $args['product_id'] = ( isset($post) && $post->post_type == 'product' ) ? $post->ID : false;
                $product_details = get_post($args['product_id']);
        }else{
                $product_details = get_post($args['product_id']);
                $args['product_id'] = ( $product_details->post_type == 'product' ) ? $post->ID : false;
        }

        if( is_null($product_details) )
                return '';

        //setup the default args
        $query_args = array(
                'orderby' => 'title',
                'order'   => 'ASC',
                'post_type'      => 'product',
                'posts_per_page' => intval($args['limit']),
                'post__not_in'   => array($args['product_id']),
                'tax_query'      => array(), //we'll add these later
        );

        //get the tags for this product
        if ( 'both' == $args['relate_by'] || 'tags' == $args['relate_by'] ) {
                $tags = get_the_terms( $args['product_id'], 'product_tag');

                if ( is_array($tags) ) {
                        foreach($tags as $tag) {
                                $tag_list[] = $tag->term_id;
                        }

                        //add the tag taxonomy query
                        $query_args['tax_query'][] = array(
                                        'taxonomy' => 'product_tag',
                                        'field' => 'id',
                                        'terms' => $tag_list,
                                        'operator' => 'IN'
                        );
                }
        }

        //are we limiting to only the assigned categories
        if( 'both' == $args['relate_by'] || 'category' == $args['relate_by'] ) {
                $product_cats = get_the_terms( $args['product_id'], 'product_category' );

                if( is_array($product_cats) ) {
                        foreach($product_cats as $cat) {
                                $categories[] = $cat->term_id;
                        }

                        $query_args['tax_query'][] = array(
                                        'taxonomy' => 'product_category',
                                        'field' => 'id',
                                        'terms' => $categories,
                                        'operator' => 'IN'
                        );
                }
        }

        //we only want to run the query if we have categories or tags to look for.
        if ( count($tag_list) > 0 || count($categories) > 0 ) {
                //make the query
                $related_query = new WP_Query($query_args);

                //how are we formatting the output
                if( $args['simple_list'] ) {

                        $output = '<div id="mp_related_products">';
                        $output .= '<div class="mp_related_products_title"><h4>' . apply_filters( 'mp_related_products_title', __('Related Products','mp') ) . '</h4></div>';
                        if( $related_query->post_count ) {
                                $list = '<ul class="mp_related_products_list">%s</ul>';
                                $items = '';
                                foreach($related_query->posts as $product) {
                                        $items .= '<li class="mp_related_products_list_item"><a href="'.get_permalink($product->ID).'">'.$product->post_title.'</a></li>';
                                }
                                $output .= sprintf($list, $items);
                        }else{
                                $output .= '<div class="mp_related_products_title"><h4>'. apply_filters( 'mp_related_products_title_none', __('No Related Products','mp') ) . '</h4></div>';
                        }

                        $output .= '</div>';
                } else {
                        //we'll use the $mp settings and functions
                        $layout_type = $mp->get_setting('list_view');
                        $output = '<div id="mp_related_products" class="mp_' . $layout_type . '">';
                        //do we have posts?
                        if( $related_query->post_count ) {
                                $output .= '<div class="mp_related_products_title"><h4>' . apply_filters( 'mp_related_products_title', __('Related Products','mp') ) . '</h4></div>';
                                $output .= $layout_type == 'grid' ? _mp_products_html_grid($related_query) : _mp_products_html_list($related_query);
                        }else{
                                $output .= '<div class="mp_related_products_title"><h4>'. apply_filters( 'mp_related_products_title_none', __('No Related Products','mp') ) . '</h4></div>';
                        }

                        $output .= '</div>';
                }
        }

        $output = apply_filters('mp_related_products', $output, $args);

        //how are we sending back the data
        if($args['echo']) {
                echo $output;
        }else{
                return $output;
        }
}
endif;