Get all meta keys assigned to a post type

Question

I need to get all meta keys/custom fields that are assigned to a post type.

I don't want to get the post_meta values for post_meta assigned to a particular post, or to all posts of a post type.

But, I want to get all possible custom fields that are 'assigned' to a post type.

I have looked and I am starting to worry that it's not possible, as maybe post_meta isn't 'registered' but only appears in the database when a post is saved?

I want to get all post meta information for a post type, in the same way I can get all taxonomies' information, assigned to a post type.

I want to be able to do:

get_post_meta_information_for_post_type($post_type);

and get something like:

array('custom_meta_key_1', 'custom_meta_key_2);

... regardless of whether or not there is even one single existing post of that post type.

Please tell me it's possible (and how to do it :))?

Thanks

Answer 1

Answer by @Gareth Gillman definitely works but its an expensive query if we have hundreds of posts.

The function below is less expensive and using the wp native function get_post_custom_keys()

if ( ! function_exists( 'us_get_meta_keys_for_post_type' ) ) :

    function us_get_meta_keys_for_post_type( $post_type, $sample_size = 5 ) {

        $meta_keys = array();
        $posts     = get_posts( array( 'post_type' => $post_type, 'limit' => $sample_size ) );


        foreach ( $posts as $post ) {
            $post_meta_keys = get_post_custom_keys( $post->ID );
            $meta_keys      = array_merge( $meta_keys, $post_meta_keys );
        }

        // Use array_unique to remove duplicate meta_keys that we received from all posts
        // Use array_values to reset the index of the array
        return array_values( array_unique( $meta_keys ) );

    }

endif;

You may use the $sample_size param to include more posts in the loop.

Hope it helps someone.

Answer 2

query all posts in the post type and then get the meta keys from the posts e.g.

Not tested, may need some amendments

$meta_fields = array();
$the_query = new WP_Query( 'post_type=posttype' );
if ( $the_query->have_posts() ) {
 while ( $the_query->have_posts() ) {
  $the_query->the_post();

  $meta_array = get_post_meta( get_the_ID() );
  foreach( $meta_array as $meta) {
   $meta_fields = $meta[];

  } 
 }
 wp_reset_postdata();
}

You can then do whatever you want with the $meta_fields variable

Answer 3

I believe that get_registered_meta_keys will do the trick. Invoke it like this, get_registered_meta_keys( 'post', $post_type ), and it'll return an associative array of all of the meta keys and the data defines them. You can call array_keys( get_registered_meta_keys( 'post', $post_type ) ) if you just want the keys.

I think that this is correct because the source for register_post_meta shows that it simply defines an 'object_subtype' key and then delegates to register_meta( 'post', $meta_key, $args ). The $object_type arg and object_subtype key that are used in register_meta seem to line-up exactly with the args that get_registered_meta_keys accepts.