JetEngine: add relation meta

function __prefix__add_relation_meta( $post_id, $meta_key, $related_posts = array() ) {

	$relation = jet_engine()->relations->get_relation_info( $meta_key );

	if ( ! $relation ) {
		return;
	}

	if ( empty( $related_posts ) ) {
		jet_engine()->relations->data->delete_all_related_meta( $meta_key, $post_id );
	}

	if ( ! is_array( $related_posts ) ) {
		$related_posts = array( $related_posts );
	}

	$prev_related    = get_post_meta( $post_id, $meta_key );
	$saved_post_type = get_post_type( $post_id );
	$type            = explode( '_to_', $relation['type'] );

	if ( $relation['post_type_1'] === $saved_post_type ) {
		$current_type = $type[0];
		$related_type = $type[1];
	} elseif ( $relation['post_type_2'] === $saved_post_type ) {
		$current_type = $type[1];
		$related_type = $type[0];
	} else {
		return;
	}

	foreach ( $related_posts as $related_post_id ) {

		if ( $related_post_id && ! in_array( $related_post_id, $prev_related ) ) {
			add_post_meta( $post_id, $meta_key, $related_post_id, false );
		}

		$stored = get_post_meta( $related_post_id, $meta_key, false );

		if ( ! is_array( $stored ) ) {
			$stored = array( $stored );
			$stored = array_filter( $stored );
		}

		if ( 'one' === $current_type ) {
			foreach ( $stored as $stored_id ) {
				if ( absint( $stored_id ) !== $post_id ) {
					delete_post_meta( $related_post_id, $meta_key );
					delete_post_meta( $stored_id, $meta_key, $related_post_id );
				}
			}
		}

		if ( $post_id && ! in_array( $post_id, $stored ) ) {
			add_post_meta( $related_post_id, $meta_key, $post_id, false );
		}

	}
}