Ok so yep it seems like you need a relationship field in both. I’m not totally comfortable with storing the relationship twice – I feel like thats a sleeper-bug waiting to happen.
ie:
– Movie A stores thats it is GenreA, GenreB, and
– GenreA stores (separately) that is has Movie A
– GenreB stores (separately) that is has Movie A
So, as a compromise, for each relation, I nominate one as the relationship owner and only it can edit the relationship. And for the non-relationship owner I have added the following meta box which will list all possible relations.
// Helper to grab all possible post types that could contain relationships
if (! function_exists('get_content_post_types')) {
function get_content_post_types() {
return array_merge(
['page', 'post'],
array_values(get_post_types([ '_builtin' => false ]))
);
}
}
// Create a query that finds our parent id in the serialised array
if (! function_exists('acf_find_parents')) {
function acf_find_parents($id, $fields) {
return new \WP_Query([
'post_type' => get_content_post_types(),
'meta_query' => [
'relation' => 'OR',
array_map(function($field) use ($id) {
return [
'key' => $field,
'value' => '"' .$id. '"',
'compare' => 'LIKE'
];
}, $fields),
]
]);
}
}
And then the usage would be something like:
// Inside the genre page
// This would find all times that this post has existed in anothers relation
acf_find_parents($post->ID, ['music', 'movies', 'books']);
An example of formatting this nicely would be:
// Using laravels collect methods
add_action('add_meta_boxes', function() {
add_meta_box('genre_relationships', 'Relationships', function($post) {
collect(acf_find_parents($post->ID, ['music', 'movies'])->posts)
->groupBy('post_type')
->each(function($type, $key) {
echo "<h3>{$key}</h3>";
$type->each(function($related) {
echo "<div>{$related->post_title}</div>";
});
});
});
});
This would spit out:
music
song G
song F
movies
movie A
movie C
Again, its a bit of a compromise, but I would prefer this over duplicating the data.
Disclaimer: The query probably should extend to join against the post table to make the post_type is actually a acf field
