Tóth Péter

How about ActiveModel::Validations?

class MyValidator
  include ActiveModel::Validations

  attr_reader :data

  validates_presence_of :name

  def initialize(data)
    @data = data || {}
  end

  def read_attribute_for_validation(key)
    data[key]
  end
end

MyValidator.new(name: '').valid? # => false

I couldn't find the etiquette guide about external links, but you can find more info about a nested case with Rails integration and tests here