Automatically fill custom field value on post publish/update

I’ve searched around a bit, and I’m having trouble finding an answer to this question. What I’m trying to do is automatically fill a custom field when a post (custom post type) is updated or published. Ideally the completed script will call out to an API and fill the custom field with the resulting information whenever a post is created or updated. For right now though, I’m just trying to automatically fill my custom field with a simple string for testing. Here’s my code:

add_action( 'save_post', 'update_tmv' );
function update_tmv($postid) {
    if ( !wp_is_post_revision( $postid ) && get_post_type( $postid ) == 'inventory') {
        $field_name = 'market_value';
        add_post_meta($postid, $field_name, 'TEST_STRING', true);    
    }
}

I’ve used this page as reference: http://www.wpbeginner.com/wp-tutorials/how-to-add-custom-fields-automatically-on-post-publish-in-wordpress/ but unfortunately it’s not working. When I publish or save a post the ‘market_value’ custom field continues to be empty (also, I’m using Advanced Custom Fields to create the custom fields). Any ideas? Thanks!

Answers:

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Method 1

See add_meta_box which has a lot of demo code for working with meta fields. Here is the most relevant part for you:

/* Do something with the data entered */
add_action( 'save_post', 'myplugin_save_postdata' );

/* When the post is saved, saves our custom data */
function myplugin_save_postdata( $post_id ) {

  // First we need to check if the current user is authorised to do this action. 
  if ( 'page' == $_POST['post_type'] ) {
    if ( ! current_user_can( 'edit_page', $post_id ) )
        return;
  } else {
    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;
  }

  $mydata = 'something'; // Do something with $mydata 

  update_post_meta( $post_id, '_my_meta_value_key', $mydata );
}

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0