unform.rb

module Unform
  extend ActiveSupport::Concern

  class FormObject
    include Virtus
    include ActiveModel::Validations
  end

  included do
    @models = {}

    class_eval do
      def self.models
        @models
      end
    end
  end

  def initialize(params = {})
    self.class.models.each do |name, model|
      instance_variable_set(:"@#{name}", model.new)

      instance_eval("def #{name}() @#{name}; end")
      instance_eval("def #{name}=(val) @#{name} = val; end")
    end

    params.each do |model, values|
      self.send(model).attributes = values
    end
  end

  def attributes
    params = {}
    self.class.models.each do |key, model|
      params[key] = self.send(key).attributes
    end
    params
  end

  def attributes=(params)
    params.each do |key, values|
      self.send(key).attributes = values
    end
  end

  def valid?
    self.class.models.any? do |name, model|
      if self.send(name).valid?
        true
      else
        false
      end
    end
  end

  module ClassMethods
    def on(model, &block)
      @models[model] ||= FormObject.class_eval(&block)
    end
  end
end

user_form.rb

class UserForm
  include Unform

  on :user do
    validates :email, presence: true
    validates :password, presence: true

    attribute :email, String
    attribute :password, String
  end
end