def convert_hash_keys(value)
  return value if (not value.is_a?(Array) and not value.is_a?(Hash))
  result = value.inject({}) do |new, (key, value)|
    new[to_snake_case(key.to_s).to_sym] = convert_hash_keys(value)
    new
  end
  result
end

def to_snake_case(string)
  string.gsub(/::/, '/').
  gsub(/([A-Z]+)([A-Z][a-z])/,'1_2').
  gsub(/([a-zd])([A-Z])/,'1_2').
  tr("-", "_").
  downcase
end

hash = {:HashKey => {:NestedHashKey => [{:Key => "value"}]}}

convert_hash_keys(hash)
# => {:hash_key => {:nested_hash_key => [{:key => "value"}]}}

def underscore_key(k)
  k.to_s.underscore.to_sym
  # Or, if you're not in Rails:
  # to_snake_case(k.to_s).to_sym
end

def convert_hash_keys(value)
  case value
    when Array
      value.map { |v| convert_hash_keys(v) }
      # or `value.map(&method(:convert_hash_keys))`
    when Hash
      Hash[value.map { |k, v| [underscore_key(k), convert_hash_keys(v)] }]
    else
      value
   end
end

hash = { camelCase: 'value1', changeMe: 'value2' }

hash.transform_keys { |key| key.to_s.underscore }
# => { "camel_case" => "value1", "change_me" => "value2" }

hash = { camelCase: 'value1', changeMe: { hereToo: { andMe: 'thanks' } } }

hash.deep_transform_keys { |key| key.to_s.underscore }
# => {"camel_case"=>"value1", "change_me"=>{"here_too"=>{"and_me"=>"thanks"}}}

hash.deep_transform_keys! do |key|
  k = key.to_s.downcase rescue key
  k.to_sym rescue key
end