custom_fields.rb

class ActiveRecord::Base  

def self.has_custom_fields(store_at, *custom_fields)
  custom_fields.each.each do |name|
    define_method name do
      self[store_at][name]
    end

    define_method "#{name}=" do |new_value|
      self[store_at][name] = new_value
    end
  end
end

mode.rb

class Contact
  has_custom_fields :custom_fields, :age, :fruta, :cor_do_cabelo
end