How to elegantly symbolize_keys for a 'nested' hash

Consider the following code:

  hash1 = {"one" => 1, "two" => 2, "three" => 3}
  hash2 = hash1.reduce({}){ |h, (k,v)| h.merge(k => hash1) }
  hash3 = hash2.reduce({}){ |h, (k,v)| h.merge(k => hash2) }
  hash4 = hash3.reduce({}){ |h, (k,v)| h.merge(k => hash3) }

hash4 is a 'nested' hash i.e. a hash with string keys and similarly 'nested' hash values.

The 'symbolize_keys' method for Hash in Rails lets us easily convert the string keys to symbols. But I'm looking for an elegant way to convert all keys (primary keys plus keys of all hashes within hash4) to symbols.

The point is to save myself from my (imo) ugly solution:

  class Hash
    def symbolize_keys_and_hash_values
      symbolize_keys.reduce({}) do |h, (k,v)|
        new_val = v.is_a?(Hash) ? v.symbolize_keys_and_hash_values : v
        h.merge({k => new_val})
      end
    end
  end

  hash4.symbolize_keys_and_hash_values #=> desired result

FYI: Setup is Rails 3.2.17 and Ruby 2.1.1

Update:

Answer is hash4.deep_symbolize_keys for Rails <= 5.0

Answer is JSON.parse(JSON[hash4], symbolize_names: true) for Rails > 5

There are a few ways to do this

1. There's a deep_symbolize_keys method in Rails

hash.deep_symbolize_keys!

2. As mentioned by @chrisgeeq, there is a deep_transform_keys method that's available from Rails 4.

hash.deep_transform_keys(&:to_sym)

There is also a bang ! version to replace the existing object.

3. There is another method called with_indifferent_access. This allows you to access a hash with either a string or a symbol like how params are in the controller. This method doesn't have a bang counterpart.

hash = hash.with_indifferent_access

4. The last one is using JSON.parse. I personally don't like this because you're doing 2 transformations - hash to json then json to hash.

JSON.parse(JSON[h], symbolize_names: true)

UPDATE:

16/01/19 - add more options and note deprecation of deep_symbolize_keys

19/04/12 - remove deprecated note. only the implementation used in the method is deprecated, not the method itself.

You cannot use this method for params or any other instance of ActionController::Parameters any more, because deep_symbolize_keys method is deprecated in Rails 5.0+ due to security reasons and will be removed in Rails 5.1+ as ActionController::Parameters no longer inherits from Hash

So this approach by @Uri Agassi seems to be the universal one.

JSON.parse(JSON[h], symbolize_names: true)

---

However, Rails Hash object still does have it.

So options are:

• if you don't use Rails or just don't care:

    JSON.parse(JSON[h], symbolize_names: true)
  

• with Rails and ActionController::Parameters:

    params.to_unsafe_h.deep_symbolize_keys
  

• with Rails and plain Hash

    h.deep_symbolize_keys
  

In rails you can create HashWithIndifferentAccess class. Create an instance of this class passing your hash to its constructor and then access it with keys that are symbols or strings (like params of Controller's Actions):

hash = {'a' => {'b' => [{c: 3}]}}

hash = hash.with_indifferent_access
# equal to:
# hash = ActiveSupport::HashWithIndifferentAccess.new(hash)

hash[:a][:b][0][:c]

=> 3

I can suggest something like this:

class Object
  def deep_symbolize_keys
    self
  end
end

class Hash
  def deep_symbolize_keys
    symbolize_keys.tap { |h| h.each { |k, v| h[k] = v.deep_symbolize_keys } }
  end
end

{'a'=>1, 'b'=>{'c'=>{'d'=>'d'}, e:'f'}, 'g'=>1.0, 'h'=>nil}.deep_symbolize_keys
# => {:a=>1, :b=>{:c=>{:d=>"d"}, :e=>"f"}, :g=>1.0, :h=>nil} 

You can also easily extend it to support Arrays:

class Array
  def deep_symbolize_keys
    map(&:deep_symbolize_keys)
  end
end

{'a'=>1, 'b'=>[{'c'=>{'d'=>'d'}}, {e:'f'}]}.deep_symbolize_keys
# => {:a=>1, :b=>[{:c=>{:d=>"d"}}, {:e=>"f"}]}

Might I suggest:

JSON.parse(hash_value.to_json)

You could use:

• Hash#to_s to convert the hash to a string;
• String#gsub with a regex to convert the keys from strings to representations of symbols; and then
• Kernel#eval to convert the string back into a hash.

This is an easy solution to your problem, but, you should only consider using it if you can trust that eval is not going to produce something nasty. If you have control over the content of the hash being converted, that should not be a problem.

This approach could be used for other kinds of nested objects, such as ones containing both arrays and hashes.

Code

def symbolize_hash(h)
  eval(h.to_s.gsub(/\"(\w+)\"(?==>)/, ':\1'))
end

Examples

symbolize_hash(hash4)
  #=> {:one=>{:one=>  {:one=>  {:one=>1, :two=>2, :three=>3},
  #                    :two=>  {:one=>1, :two=>2, :three=>3},
  #                    :three=>{:one=>1, :two=>2, :three=>3}},
  #           :two=>  {:one=>  {:one=>1, :two=>2, :three=>3},
  #                    :two=>  {:one=>1, :two=>2, :three=>3},
  #                    :three=>{:one=>1, :two=>2, :three=>3}},
  #           :three=>{:one=>  {:one=>1, :two=>2, :three=>3},
  #                    :two=>  {:one=>1, :two=>2, :three=>3},
  #                    :three=>{:one=>1, :two=>2, :three=>3}}},
  #    :two=>{:one=>  {:one=>  {:one=>1, :two=>2, :three=>3},
  #    ...
  #    :three=>{:one=>{:one=>  {:one=>1, :two=>2, :three=>3},
  #    ...
  #                    :three=>{:one=>1, :two=>2, :three=>3}}}}

symbolize_hash({'a'=>1, 'b'=>[{'c'=>{'d'=>'d'}}, {e:'f'}]})
  #=> {:a=>1, :b=>[{:c=>{:d=>"d"}}, {:e=>"f"}]}

Explanation

(?==>) in the regex is a zero-width postive lookahead. ?= signifies positive lookahead; => is the string that must immediately follow the match to \"(\w+)\". \1 in ':\1' (or I could have written ":\\1") is a string beginning with a colon followed by a backreference to the content of capture group #1, the key matching \w+ (without the quotes).