Create recursive OpenStruct from a Ruby Hash

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Maybe you already know and used the Ruby's OpenStruct class. I've played a bit with it to create a very easy to use configuration system for Ruby apps.


When I develop some ruby app, mostly rails ones, I need to manage several configuration variables (eg: API keys) and I like to store them in a unique place. So I usually end up with something like this:

# config/initializers/_load_config.rb

path = File.read("#{Rails.root}/config/config.yml")

APP_CONF = ActiveSupport::HashWithIndifferentAccess.new(
  YAML.load(ERB.new(path).result)[Rails.env]
)

It does a good job:

  • it first compiles eventual ERB tags (so you can store values in ENV)
  • then it loads YAML
  • finally initializes a HashWithIndifferentAccess, so I can lookup keys with either a String or a Symbol

The last point is a plus, kindly offered by Rails' ActiveSupport library, I prefer Symbol over String for Hash keys.

The problem

However, I knew that there's a smarter way to achieve same goal, even with some more adavantages. I started from OpenStruct:

  • it solves the problem of Hash key lookup by providing them as methods, for example:
require 'ostruct'

hash = {a: 1, b: 2}
mystruct = OpenStruct.new hash
mystruct.a # => 1
mystruct.b # => 2
  • it's part of the Ruby's standard library (along with YAML and ERB as well)
  • it's very simple to hack ;-)

Looks excellent, right? Well, it's not perfect, because it doesn't handle nested hashes, in other words, here's what happen when you pass an Hash that has other hashes as values:

require 'ostruct'

hash = {a: {b: 1}}
mystruct = OpenStruct.new hash
mystruct.a # => {b: 1}
mystruct.a.b # => NoMethodError: undefined method `b' for {:b=>1}:Hash

Moreover, there's no way to access the original hash (well, there's one but that's another point), it might be useful when you need to pass an entire Hash.

A solution

OpenStruct already does a great part fo the dirty job, it only needs some change to get what we need. I called it, DeepStruct:

# deep_struct.rb
require 'ostruct'

class DeepStruct < OpenStruct
  def initialize(hash=nil)
    @table = {}
    @hash_table = {}

    if hash
      hash.each do |k,v|
        @table[k.to_sym] = (v.is_a?(Hash) ? self.class.new(v) : v)
        @hash_table[k.to_sym] = v

        new_ostruct_member(k)
      end
    end
  end

  def to_h
    @hash_table
  end

end

There's not too much to explain here, I've overridden OpenStruct#new method to make two basic things:

  • Iterate the key/values of the passed Hash, then:
    • initialize a new DeepStruct if a given value is an Hash
    • store original key/values in @hash_table, so I can retrieve it with #to_h method

That's all we need, here's an example:

require 'deep_struct'

hash = {a: {b: 1}}
mystruct = DeepStruct.new hash
mystruct.a # => #<DeepStruct b=1>
mystruct.a.b # => 1
mystruct.a.to_h # => {b: 1}

Conclusion

I've said I'd mainly use this to store configs for my apps, so go back to the initial example:

# config/initializers/_load_config.rb

path = File.read("#{Rails.root}/config/config.yml")

APP_CONF = DeepStruct.new(YAML.load(ERB.new(path).result)[Rails.env])

Enjoy and/or leave a comment :-)

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