Using factory_girl with mongoid to test referenced_in/references_many

I am trying to test an associated document for a subscription service. Each subscription is embedded in an account and references a plan. Below is the various bits of code:

The account:

Factory.define :account, :class => Account do |a|
  a.subdomain 'test'
  a.agents { [ Factory.build(:user) ] }
  a.subscription { Factory.build(:free_subscription) }
end

The subscription:

Factory.define :free_subscription, :class => Subscription do |s|
  s.started_at Time.now
  s.plan { Factory.build(:free_plan) }
end

The plan:

Factory.define :free_plan, :class => Plan do |p|
  p.plan_name 'Free'
  p.cost 0
end

The error:

Mongoid::Errors::InvalidCollection: Access to the collection for Subscription is not allowed since it is an embedded document, please access a collection from the root document.

If I comment out the line that links the plan to the subscription then the tests work, but obviously I can't test that the subscription has a plan.

Any suggestions would be greatly appreciated.

UPDATE:

Here are the models:

class Account
  include Mongoid::Document

  field :company_name, :type => String
  field :subdomain, :type => String
  field :joined_at, :type => DateTime

  embeds_one :subscription

  accepts_nested_attributes_for :subscription

  before_create :set_joined_at_date

  private

  def set_joined_at_date
    self.joined_at = Time.now
  end
end

class Subscription
  include Mongoid::Document

  field :coupon_verified, :type => Boolean
  field :started_at, :type => DateTime

  referenced_in :plan
  embedded_in :account, :inverse_of => :subscription
end

class Plan
  include Mongoid::Document

  field :plan_name, :type => String
  field :cost, :type => Integer
  field :activ开发者_运维问答e_ticket_limit, :type => Integer
  field :agent_limit, :type => Integer
  field :company_limit, :type => Integer
  field :client_limit, :type => Integer
  field :sla_support, :type => Boolean
  field :report_support, :type => Boolean

  references_many :subscriptions
end

You need to create an account with the subscription for it to be valid.

Factory.define :free_subscription, :class => Subscription do |s|
  s.started_at Time.now
  s.plan { Factory.build(:free_plan) }
  s.account { Factory(:account) }
end