Advanced Rails - Building Industrial-Strength Web Apps in Record Time

Hash

68 | Chapter 2: ActiveSupport and RailTies

h = {:a => 123, "b" => 456}

h.stringify_keys # => {"a"=>123, "b"=>456}

h.symbolize_keys # => {:a=>123, :b=>456}

• Hash#assert_valid_keys(:key1, ...)raises anArgumentErrorif the hash contains keys
  not in the argument list. This is used to ensure that only valid options are provided to
  a keyword-argument-based function.


• Hash#reverse_mergeandHash#reverse_merge!(in-place) work likeHash#merge, but in
  reverse order; in the case of duplicate keys, existing tuples beat those in the argument.
  This can be used to provide default arguments to a function.
  options = {:a => 3, :c => 5}
  options.reverse_merge!(:a => 1, :b => 4)
  options # => {:a=>3, :b=>4, :c=>5}


• Hash#slicereturns a new hash with only the keys specified. Inversely,Hash#except
  returns a new hash excluding the specified keys:
  options = {:a => 3, :b => 4, :c => 5}
  options.slice(:a, :c) # => {:c=>5, :a=>3}
  options.except(:a) # => {:b=>4, :c=>5}
  Hash#sliceandHash#exceptalso come in in-place versions, respectively:Hash#slice!
  andHash#except!.

HashWithIndifferentAccess core_ext/hash/indifferent_access.rb

AHashWithIndifferentAccessis a hash whose elements can be accessed with either string
or symbol keys:

options = {:a => 3, :b => 4}.with_indifferent_access

options.class # => HashWithIndifferentAccess

options[:a] # => 3

options["a"] = 100

options[:a] # => 100

HashWithIndifferentAccessis primarily used to offer a nice API to users, for example, so
that a developer can write params[:user]or params["user"]. The stringify_keys or
symbolize_keys methods should be used in option processing.

However,HashWithIndifferentAccessalso fixes a class of security exploits in Rails. In Ruby
1.8, symbols are not garbage collected, so aparamshash purely keyed on symbols (as it
used to be) could lead to a denial-of-service attack. A malicious client could leak memory
and exhaust the symbol table by issuing many requests with unique parameter names. Inci-
dentally, Ruby 1.9 will obviate the need forHashWithIndifferentAccess, because symbols
and strings will be treated identically (:aoeu == 'aoeu').