This post originated from an RSS feed registered with Ruby Buzz by Guy Naor. Original Post: Beware of HashWithIndifferentAccess#symbolize_keys! Feed Title: Famundo - The Dev Blog Feed URL: http://devblog.famundo.com/xml/rss/feed.xml Feed Description: A blog describing the development and related technologies involved in creating famundo.com - a family management sytem written using Ruby On Rails and postgres Latest Ruby Buzz Posts Latest Ruby Buzz Posts by Guy Naor Latest Posts From Famundo - The Dev Blog

While working on our Famundo application, we used the symbolize_keys! method of one of the Hashs in rails, not realizing it was actually a HashWithIndifferentAccess. To our amazement, all the entries in the hash just disappeared.

Not ones to leave alone an inexplicable event, we decided to research it a bit. Time to dig into the rails code for Hash. This is the code used to symbolize the keys (in keys.rb):

    # Destructively convert all keys to symbols.
    1 def symbolize_keys!
    2   keys.each do |key|
    3     unless key.is_a?(Symbol)
    4       self[key.to_sym] = self[key]
    5       delete(key)
    6     end
    7   end
    8   self
    9 end

The source of the problem is that in a HashWithIndifferentAccess a symbol and a string are treated as one and the same, by converting the symbol into a string on assignment to the hash. Line 3 will be true for every key in the hash entered as either a symbol (that was converted to a string) or a string. The assignment in line 4 will actually assign the value to itself, and then line 5 will delete our key, leaving us with an empty hash. The best way to see it is to launch script/console, and enter the following:

    >> h = HashWithIndifferentAccess.new
    => {}
    >> h[:first] = "Item 1"
    => "Item 1"
    >> h['second'] = "Item 2"
    => "Item 2"
    >> h
    => {"second"=>"Item 2", "first"=>"Item 1"}
    >> h.symbolize_keys!
    => {}
    >> h
    => {}

The fix is pretty simple and involves making sure that we don't try to run symbolize_keys! on a HashWithIndifferentAccess. So the new method will now be:

    # Destructively convert all keys to symbols.
    def symbolize_keys!
      unless self.kind_of? HashWithIndifferentAccess # This is the added check
        keys.each do |key|
          unless key.is_a?(Symbol)
            self[key.to_sym] = self[key]
            delete(key)
          end
        end
      end
      self
    end

I submitted a ticket with a patch to the Rails trac. I hope it'll be included soon so that nobody else get bitten by this bug.