Include woocommerce custom field value in front-end search result

currently the search result only showing all the woocommerce default product field (product title, category, tag, .etc).

I’ve add a new custom field inside the woocommerce product using this code inside functions.php

/** Add Sub-Title option in Woocommerce */

add_action( 'woocommerce_product_options_general_product_data', 'my_custom_field' );

function my_custom_field() {

woocommerce_wp_text_input( 
array( 
    'id'          => '_subtitle', 
    'label'       => __( 'Reference', 'woocommerce' ), 
    'placeholder' => 'Reference code',
    'description' => __( 'Enter the reference code', 'woocommerce' ) 
)
);

}

add_action( 'woocommerce_process_product_meta', 'my_custom_field_save' );

function my_custom_field_save( $post_id ){  

$subtitle = $_POST['_subtitle'];
if( !empty( $subtitle ) )
    update_post_meta( $post_id, '_subtitle', esc_attr( $subtitle ) );

}

How can I include this custom field value in the search result? I’ve tried google for it, but no any significant solution.

You have to modify WooCommerce search query.

1. Hook into this action: do_action( 'woocommerce_product_query', $query, $instance );
2. Detect if it is a search query
3. Add your _subtitle meta

Something like this (not tested):

if ( $query->is_search ) {
    $query->set('meta_query', array(
        array(
            'key' => '_subtitle',
            'value' => $query->query_vars['s'],
            'compare' => 'LIKE'
        )
    ));
};