code: function _getStopEdges(db, firstId, callback) {
  
    // Check cache for edges
    let cacheKey = db.id + "-" + firstId;
    let cache = cache_edges.get(cacheKey);
    if ( cache !== null ) {
      return callback(null, cache);
    }
  
    // Build Select Statement
    let select = "SELECT stop2_id FROM rt_line_graph WHERE stop1_id='" + firstId + "';";
  
    // Query the database
    db.select(select, function(err, results) {
  
      // Database Query Error
      if ( err ) {
        return callback(err);
      }
  
      // List of edges to return
      let rtn = [];
  
      // Add each connected stop
      for ( let i = 0; i < results.length; i++ ) {
        rtn.push(results[i].stop2_id);
      }
  
      // Return edges
      cache_edges.put(cacheKey, rtn);
      return callback(null, rtn);
  
    });
  
  }
docs: Get the Line Graph Edges (next Stop ID, direction, and weight)
  for the specified Stop
  @param {RightTrackDB} db The Right Track Database to query
  @param {String} firstId The first Stop ID
  @param {function} callback Callback Function(err, edges)
  @private