// publish our nodes.
function publish_nodes(){
  var nodeCursor = nodeCollection.find( { _id: {$gt: 0}} );
  nodeCursor.each(function (err, doc) {
    if (err) {
      console.log(err);
    } else if ( doc!= null) {
      //console.log('Fetched:', doc);
      var node_to_publish = doc;

      // Get sensors for this node.
      var sensor_cursor = sensorCollection.find( {node_id : node_to_publish['_id']} );
      var sensor = sensor_cursor.each(function ( serr, sdoc ){
        if (serr){
          console.log(err);
        } else if (sdoc!= null){
          //console.log('fetched sensor for node: ', node_info_json['_id'] ,":", sdoc);
          console.log( sdoc );
        }
      });

      //node_to_publish
      console.log('published node.');
      mqtt_client.publish("/zc/" + serial_number + "/node/", JSON.stringify(node_to_publish) );
    }
  });
}