getting index for an Om component

component_index.cljs

(defn piece-view [data owner]
  (let [[_ row] (om/path data)]
    (reify          
      om/IRender    
      (render [_]    
        (dom/div nil row)))))
(defn board-view [app owner]
  (let [size  40]
    (reify
      om/IRender
      (render [_]
        (apply
          dom/div
          #js {
               :style #js { :width size :height size }
               :id "board"
               }
       (om/build-all piece-view [ 3 3 3 3 ] ))))))