add meta box considers the selected option as post parent

Actually the problem is that i was giving the select option a name of “parent_id”, which for some how automatically used as post parent, but once i changed it to other name it worked.

Working code:

<?php
function hfx_register_meta_boxes() {
add_meta_box( 'select_hfx_workshop', __( 'Workshop for', HFX_DOMAIN ), 'hfx_select_workshop_parent_meta_box', 'forum', 'side','core');
}
add_action( 'add_meta_boxes_forum', 'hfx_register_meta_boxes' );


function hfx_select_workshop_parent_meta_box(){
    global $post;

    $selected = '';

    wp_nonce_field( 'select_workshophfx_box_action', 'select_workshophfx_box' );

    if(get_post_meta( $post->ID, 'workshop_id', true )){
        $selected = get_post_meta( $post->ID, 'workshop_id', true );
    }

    if(post_type_exists( 'product' )){
        $workshopsfx = get_posts(
        array(
            'post_type'   => 'product',
            'orderby'     => 'title',
            'order'       => 'ASC',
            'numberposts' => -1
        )
    );

    if ( !empty( $workshopsfx ) ) {


        echo '<select name="workshop_id" class="widefat">';
        echo '<option value="">'.esc_html__('Choose a workshop', HFX_DoMAIN).'</option>';
        foreach ( $workshopsfx as $workshop ) {
            printf( '<option value="%s"%s>%s</option>', esc_attr( $workshop->ID ), selected( $workshop->ID, $selected, false ), esc_html( $workshop->post_title ) );
        }

        echo '</select>';
    }
    }
}

function hfx_save_meta_box( $post_id ) {
if ( !isset( $_POST['select_workshophfx_box'] ) || !wp_verify_nonce( $_POST['select_workshophfx_box'], 'select_workshophfx_box_action' ) ){
    return;
}

if ( ! current_user_can( 'edit_post', $post_id ) ){
    return;
}


if ( $parent_id = wp_is_post_revision( $post_id ) ) {
    $post_id = $parent_id;
}
$fields = [
    'workshop_id',
];
foreach ( $fields as $field ) {
    if ( array_key_exists( $field, $_POST ) ) {
        update_post_meta( $post_id, $field, sanitize_text_field( $_POST[$field] ) );
    }
 }
}