code: function readJointWithBall(conn, unit, handleJoint) {
  	readJoint(conn, unit, {
  		ifNotFound: function(){
  			throw Error("joint not found, unit "+unit);
  		},
  		ifFound: function(objJoint){
  			if (objJoint.ball)
  				return handleJoint(objJoint);
  			conn.query("SELECT ball FROM balls WHERE unit=?", [unit], function(rows){
  				if (rows.length === 1)
  					objJoint.ball = rows[0].ball;
  				handleJoint(objJoint);
  			});
  		}
  	});
  }
docs: add .ball even if it is not retrievable