MemoizedHelpers is based on the implementation of #let in Rspec found here: https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/memoized_helpers.rb. When I run this I get NoMethodError super: no superclass method `tenant' for #<RegistrationForm:0x007fb25f0c3a78> at Line 28 of memoized_helpers.rb

base.rb

class FormObject::Base
  include ActiveModel::Model
  include MemoizedHelpers

  attr_reader :params, :errors

  def initialize(params = {})
    @params = ActionController::Parameters.new(params)
    @errors = ActiveModel::Errors.new(self)
  end

  def save
    valid? && persist
  end
end

memoized_helpers.rb

module FormObject
  module MemoizedHelpers
  private
    def __memoized
      @__memoized ||= {}
    end

    def self.included(mod)
      mod.extend(ClassMethods)
    end

    def self.definitions_module_for(form_object)
      if form_object.const_defined?(:ModelDefinitions, false)
        form_object.const_get(:ModelDefinitions, false)
      else
        form_object.const_set(:ModelDefinitions, Module.new)
      end
    end

    module ClassMethods
      def exposing(name, &block)
        raise "#exposing called without a block" unless block_given?

        MemoizedHelpers.definitions_module_for(self).send(:define_method, name, &block)

        # If the exposing block has a parameter set it to the FormObject instance. I'd
        # rather not do this at all and just always execute in the correct context.
        # eg: remove the block.arity conditional and just expect arity to be 0.
        if block.arity == 1
          define_method(name) { __memoized.fetch(name) { |k| __memoized[k] = super(self, &nil) } }
        else
          define_method(name) { __memoized.fetch(name) { |k| __memoized[k] = super(&nil) } }
        end
      end
    end
  end
end

registration_form.rb

class RegistrationForm < FormObject::Base
  # Instead of having to specify |form_instance| I would like the block
  # to be automatically executed in the context of the form_instance
  # so I can do something like User.new(user_params) instead of form_instance.user_params
  exposing(:user)   { |form_instance| User.new(form_instance.user_params) { |u| u.is_admin = true } }
  exposing(:tenant) { |form_instance| user.build_tenant(form_instance.tenant_params) }

  # Ultimately, the #expsosing calls will dynamically create these validate blocks
  validate do
    tenant.errors.each do |key, value|
      errors.add("#{tenant.class.name.underscore}_#{key}", value)
    end unless tenant.valid?
  end

  validate do
    user.errors.each do |key, value|
      errors.add("#{user.class.name.underscore}_#{key}", value)
    end unless user.valid?
  end

  private

  def persist
    user.save
  end

  def user_params
    params.fetch(:user, {}).permit(:first_name, :last_name, :email, :password, :password_confirmation)
  end

  def tenant_params
    params.fetch(:tenant, {}).permit(:name)
  end
end