Polymorphic embedding in a list

poly_embed.rb

require "mongoid"
require "minitest/autorun"

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('test_polymorphic_embedding')
end

class Condition; end

# A relation consists of a name and a list of arguments
#
# e.g. agent(e, x)
# The name is `agent' and the arguments are `e' and `x'
#
# Relations are one kind of condition.
class Relation < Condition
  include Mongoid::Document

  field :name

  # need to change this, so that a relation can also be stored
  # in the arguments array
  embeds_many :arguments, class_name: 'StringArgument'
end

class StringArgument
  include Mongoid::Document

  field :name
end

class RelationModellingTest < MiniTest::Unit::TestCase

  # agent(e, x)
  def test_build_a_relation_with_string_arguments_from_a_hash
    input = { '_type'     => 'Relation',
              'name'      => 'agent',
              'arguments' => [
                { '_type' => 'StringArgument',
                  'name'  => 'e' },
                { '_type' => 'StringArgument',
                  'name'  => 'x' }
              ] }
    relation = Relation.new(input)

    assert_kind_of StringArgument, relation.arguments.first
    assert_kind_of StringArgument, relation.arguments.last
  end

  # copy(direction(x), direction(y))
  def test_create_a_nested_relation
    first_arg = { '_type'     => 'Relation',
                  'name'      => 'direction',
                  'arguments' => [
                    { '_type' => 'StringArgument',
                      'name'  => 'x' }
                  ] }

    second_arg = { '_type'     => 'Relation',
                   'name'      => 'direction',
                   'arguments' => [
                     { '_type' => 'StringArgument',
                       'name'  => 'x' }
                   ] }

    input = { '_type'     => 'Relation',
              'name'      => 'copy',
              'arguments' => [first_arg, second_arg] }

    relation = Relation.new(input)
    assert_kind_of Relation, relation.arguments.first
    assert_kind_of Relation, relation.arguments.last
  end

end

good,thank you