Grape length validator

length.rb

class Length < Grape::Validations::SingleOptionValidator
  def validate_param!(attr_name, params)
    case @option
    when Integer
      unless params[attr_name].to_s.length == @option
        throw :error, :status => 400,
          :message => "#{attr_name} must be #{@option} characters long"
      end
    when Range
      unless @option.include?(params[attr_name].to_s.length)
        throw :error, :status => 400,
          :message => "#{attr_name} must be between #{@option.begin} and #{@option.end} characters long"
      end
    end
  end
end