gistfile1.rb

class Tree

  def initialize(name = nil, path = '')
    @name = name
    @path = path
    @folders = {}
    @files = []
  end

  attr_reader :name, :path, :folders, :files

  def add_file(obj)
    file = extract_data(obj)
    folder_for(file['path']).files << file
  end

  def to_h
    if @name
      {
        "name" => @name,
        "isDir" => true,
        "path" => @path,
        "children" => children
      }
    else
      children
    end
  end

  private

  def children
    @folders.values.map(&:to_h) + @files
  end

  def extract_data(obj)
    {
      "name" => File.basename(obj[:key]),
      "isFile" => true,
      "path" => obj[:key],
      "meta" => {
        "last_modified" => obj[:last_modified].utc.strftime('%Y-%m-%dT%H:%M:%S+00:00'),
        "content_type" => nil, # must call #head_object to get this or guess it
        "content_length" => obj[:size].to_i,
      }
    }
  end

  def folder_for(path)
    File.dirname(path).split('/').inject(self) do |tree, folder_name|
      tree.folders[folder_name] ||= Tree.new(folder_name, tree.path + folder_name + '/')
    end
  end

end

def process_keys(resp, tree)
  resp[:contents].each do |obj|
    tree.add_file(obj)
  end
end

options = { bucket_name: 'my-bucket-name' }

s3 = AWS::S3::Client.new
resp = s3.list_objects(options)

tree = Tree.new
process_keys(resp, tree)

# you must request the next page of results until you have all objects
while resp[:truncated]
  resp = s3.list_objects(options.merge(:marker => resp[:contents].last[:key]))
  process_keys(resp, tree)
end

tree.to_h