update custom field if blank - wordpress

Question

need to update a custom field value if it is blank mis if it has any value not want to do anything
I want to update only if it is empty
code:
$custom_fields = get_post_custom($post_ID);//Current post id
$my_custom_field = $custom_fields['artist'];//key name
foreach ( $my_custom_field as $key => $value );
if(empty($value) or ($value == '')){
$artist_v = get_post_custom_values('artist', $post_ID);
update_post_meta($post_ID, 'artist', $parent_title, $artist_v);
}else{
//add_post_meta($post_ID, 'artist', $parent_title, true);
}

Answer 1

Below example might help:
`
if(!empty($ARR_meta_data))
{
foreach($ARR_meta_data as $meta)
{
if(trim($meta['meta_value']) == "")
{
update_postmeta_by_meta_id($meta['meta_id'], " YOUR CUSTOM VALUE ");
}
}
}
function get_postmeta_details_in_array($post_id, $meta_key)
{
global $wpdb;
$sql = "SELECT * FROM ".$wpdb->prefix."postmeta
WHERE meta_key='{$meta_key}' AND post_id='{$post_id}'";
return $wpdb->get_results($sql, ARRAY_A);
}
function update_postmeta_by_meta_id($meta_id, $meta_value)
{
global $wpdb;
$sql = "UPDATE ".$wpdb->prefix."postmeta SET meta_value='$meta_value'
WHERE meta_id='$meta_id'";
return $wpdb->query($sql);
}
?>
`
Place get_postmeta_details_in_array() and update_postmeta_by_meta_id() function in the functions.php in your wordpress theme.