apply_filters( 'mastodon_api_relationship', function ( $relationship, $user_id, $request ) {
     $user = get_user_by( 'ID', $user_id );

     $relationship     = new Entity\Relationship();
     $relationship->id = strval( $user->ID );

     return $relationship;
}, 10, 3 );

apply_filters( 'mastodon_api_relationship', null, $user_id, $request )