FactoryGirl 属于并具有 has_many 关联

Question

我有三个类：

class User
  has_and_belongs_to_many :accounts
end

class Account
  has_and_belongs_to_many :users
end

class Idea
  belongs_to :user
  belongs_to :account
end

这些工厂：

FactoryGirl.define do
  sequence :subdomain do |n|
    "subdomain#{n}"
  end

  factory :account do
    subdomain  { FactoryGirl.generate(:subdomain) }
  end

end

FactoryGirl.define do
  sequence :email do |n|
    "foo-#{n}@example.com"
  end

  sequence :name do |n|
    "Test User #{n}"
  end

  factory :user do
    name  { FactoryGirl.generate(:name) }
    email { FactoryGirl.generate(:email) }
    password "secret"
    after_create {|i| Factory(:account, :user_ids => [i.id])}
  end

end

Factory.define :idea do |idea|
  idea.title 'Lorem ipsum dolor'
  idea.description  'Lorem ipsum dolor sit amet'
  idea.sequence(:user_id) { |n| Factory(:user).id }
  idea.sequence(:account_id) { |n| idea.user.accounts.first.id }
end

最后一个不起作用，并引发以下错误：

Failure/Error: idea = Factory :idea
     NoMethodError:
       undefined method `accounts' for #<FactoryGirl::Declaration::Implicit:0x007fcc6c370a60>

如何分解创意和关联用户的（其中一个）帐户之间的关联？