Preserve term order per post in a non-hierarchical taxonomy

Question

I'm looking for the simplest method of preserving the term order in a non-hierarchical taxonomy on a per-post basis. I was able to achieve this previously using the following code in register_taxonomy coupled with a get_the_terms filter.

'sort' => true,
'args' => array( 'orderby' => 'term_order' ),

As of WordPress 4.7 this no longer seems to work, and the sort argument no longer appears to do anything at all.

All I need is to be able to enter "bananas, apples, pears" in a "Fruits" taxonomy field on the Add/Edit Post page and have that term order preserved after I press "Publish" or "Update." The next post could list "pears, bananas, apples". Right now even if I can get that original order to stick after the initial Publish, because it alphabetizes it on the admin page any original order is lost on subsequent publishes/updates to the Post.

Is there a filter I can apply to prevent the values from being reordered alphabetically, or does the actual term order need to be saved as separate metadata?

For reference, the code I had originally used (which I've seen reused here to answer similar questions) is at the bottom of this page: https://core.trac.wordpress.org/ticket/5857 and term_order is discussed in further detail here: https://core.trac.wordpress.org/ticket/9547

Answer 1

I encountered a similar issue and this is how I solved it. Note that I put in Fruits as the taxonomy name based on your example. It's important that this name matches the taxonomy name exactly. Often taxonomy names are singular, i.e. fruit

function wp4382_filter_terms_order( $orderby, $query_vars, $taxonomies ) {
    if ( ! empty( $query_vars['object_ids'] )
        && ! empty( $taxonomies[0] )
        && ( 1 === count( $taxonomies ) )
        && 'Fruits' === $taxonomies[0] )
    {
        $orderby = 'tr.term_order';
    }
    return $orderby;
}

add_filter( 'get_terms_orderby', 'wp4382_filter_terms_order', 10, 3 );