Flatten an arbitrarily nested hash into a flat hash with dotted key names

flatten_hash_recurse.rb

a = { a: 1, b: { c: 2, d: { e: 3 }}}

# define a recursive proc:
flatten_keys = -> (h, prefix = "") do
  @flattened_keys ||= {}
  h.each do |key, value|
    # Here we check if there's "sub documents" by asking if the value is a Hash
    # we also pass in the name of the current prefix and key and append a . to it
    if value.is_a? Hash
      flatten_keys.call value, "#{prefix}#{key}."
    else
      # if not we concatenate the key and the prefix and add it to the @flattened_keys hash 
      @flattened_keys["#{prefix}#{key}"] = value
    end
  end
  @flattened_keys
end

flattened = flatten_keys.call a