PDOException using ->join

Of course, I could be doing it wrong as this is first time I've attempted to use a Join but I've tried to follow your examples. Seems to be looking for an array where it expects a column.

I'd just like the joined data for display and search, it doesn't need to be actually edited like the primary fields.

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[42S22]: Column not found: 1054 Unknown column 'status_reporting.Array' in 'field list'' in /blah/system/dataTables/Database/Driver/Mysql/Query.php:98 

Tables are:

status_reporting (containing field rock_id, which I wish to join to the child's "id" field)
big_rocks (child, keyed on "id")

Code looks like this:

require_once( "../system/dataTables/DataTables.php" );
use
  DataTables\Editor,
  DataTables\Editor\Field,
  DataTables\Editor\Format,
  DataTables\Editor\Join,
  DataTables\Editor\Validate;

  Editor::inst( $db, 'status_reporting' )
  ->field(
    Field::inst( 'action_date' ) ->validator( 'Validate::dateFormat', 'Y-m-d' ),
    Field::inst( 'rock_id' )     ->validator( 'Validate::required' ),
    Field::inst( 'author_id' )   ->validator( 'Validate::required' ),
    Field::inst( 'state' ),
    Field::inst( 'thecomment' )  ->validator( 'Validate::required' )
  )
  ->join(
    Join::inst( 'big_rocks', 'object' )
      ->join( 
          array( 'rock_id', 'id' )  
        )
      ->field( 
          Field::inst( 'team_id' ) 
        )
    )
  ->process( $_POST )
  ->json();

Many thanks for any response, it'd be great to get this working to save me having loads of reference arrays in JavaScript.

Iain