# File lib/mongo/gridfs/grid.rb, line 15
    def initialize(db, fs_name=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

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