# File lib/mongo/gridfs/grid_file_system.rb, line 14
    def initialize(db, fs_name=Grid::DEFAULT_FS_NAME)
      raise MongoArgumentError, "db must be a Mongo::DB." unless db.is_a?(Mongo::DB)

      @db      = db
      @files   = @db["#{fs_name}.files"]
      @chunks  = @db["#{fs_name}.chunks"]
      @fs_name = fs_name

      @default_query_opts = {:sort => [['filename', 1], ['uploadDate', -1]], :limit => 1}

      # This will create indexes only if we're connected to a primary node.
      connection = @db.connection
      begin
        @files.ensure_index([['filename', 1], ['uploadDate', -1]])
        @chunks.ensure_index([['files_id', Mongo::ASCENDING], ['n', Mongo::ASCENDING]], :unique => true)
      rescue Mongo::ConnectionFailure
      end
    end