A simple way to use has_many between form objects using Virtus.The problem I had was to get validations working with fields_for in the view, but still have the flexibility and full control using virtus instead of active record models.And I added a way of checking for sanitised args in rails 4.

album_form.rb

# encoding: utf-8

class AlbumForm < BaseForm

  has_many :songs, class_name: 'SongForm'
  
  validates :songs, form_collection: true
  
end

base_form.rb

# encoding: utf-8

# The goal with this base form is to keep the child classes
# as clean as possible

class BaseForm
  include Virtus.model
  include ActiveModel::Validations
  include HasManyAssociation
  include ActiveModel::ForbiddenAttributesProtection
  
  def initialize(*args, &block)
    sanitize_args args
    super
  end
  
  def persisted?
    false
  end

  def save
    if valid?
      persist!
      true
    else
      false
    end
  end

  def persist!
    raise "Implement me!"
  end
  
  protected
    def sanitize_args(args)
      args.each do |value|
        sanitize_for_mass_assignment(value)
      end
    end
  
end

destroyable.rb

# encoding: utf-8

# This could be needed if you whant to support :_destroy via the view
# Example of using destroy is rails cast #403
module Destroyable
  extend ActiveSupport::Concern
  
  # It's used in conjuction with fields_for.
  # See ActionView::Helpers::FormHelper::fields_for for more info.
  def _destroy
    marked_for_destruction?
  end
  
  def marked_for_destruction?
    @marked_for_destruction
  end

  def mark_for_destruction
    @marked_for_destruction = true
  end

end

form_collection_validator.rb

# encoding: utf-8

class FormCollectionValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, collection)
    collection.each do |form|
      validate_single_form(attribute, record, form)
    end
  end

  def validate_single_form(name, record, form)
    unless valid = form.valid?
      form.errors.each do |attribute, message|
        attribute = "#{name}.#{attribute}"
        record.errors[attribute] << message
        record.errors[attribute].uniq!
      end
    end
    valid
  end
end

has_many_association.rb

# encoding: utf-8

module HasManyAssociation
  extend ActiveSupport::Concern
  
  protected
  
  # Determines if a hash contains a truthy _destroy key.
  def has_destroy_flag?(hash)
    ActiveRecord::ConnectionAdapters::Column.value_to_boolean(hash['_destroy'])
  end
  
  module ClassMethods
  
    def has_many(name, options = {})
      raise ArgumentError, "association names must be a Symbol" unless name.kind_of?(Symbol)
      
      klass = extract_class_name(options) || calculate_class_name(name)
      attribute name, Array[klass]
    
      # This is a requirent in order to get fields_for working in the view.
      define_method "#{name}_attributes=" do |attributes|
        send("#{name}=",[]) unless send(name)
        attributes.each do |k,v|
          obj = klass.new(v)
          send(name) << obj if obj.persisted? || !has_destroy_flag?(v)
        end
      end
    end
    
    def calculate_class_name(name)
      "#{name.singularizee}_form".classify.constantize
    end

    def extract_class_name(options)
      options[:class_name] and options[:class_name].constantize
    end

    def association_class(name)
      if association = attribute_set.detect{ |a| a.name == name }
        association.type.member_type
      end
    end
  end
end

song_form.rb

# encoding: utf-8

class SongForm < BaseForm
  include Destroyable
  
  attribute :name, String
  
  validates :name, presence: true

end

If you are looking at Ryan Bates episode about dynamic forms beware that the following will not work.

f.object.send(association).klass.new

Instead use the function association_class that's defined in HasManyAssociation

f.object.class.association_class(association).new