rom-mapper.rb

require 'rom-mapper'
require 'json'

class ProfileMapper < ROM::Mapper
  reject_keys true
  attribute :id, from: "userId"
  unwrap "profile" do
    attribute :created_at, from: "firstDate"
    # attribute :metric
    # attribute :locale
    unwrap "localization" do
      attribute :metric, from: "metric"
      attribute :locale, from: "locale"
    end
  end
end

json = JSON.parse([
  {
    userId: 1,
    profile: {
      firstDate: '2012-12-12',
      localization: {
        metric: true,
        locale: 'en'
      }
    }
  }
].to_json)

puts ProfileMapper.build.call(json)
# {:id=>1, "profile"=>{:metric=>true, :locale=>"en"}, :created_at=>"2012-12-12"}