How to set up factory in FactoryGirl with has_many association

Can someone tell me if I'm just going about the setup the wrong way?

I have the following models that have has_many.through associations:

class Listing < ActiveRecord::Base
  attr_accessible ... 

  has_many :listing_features
  has_many :features, :through => :listing_features

  validates_presence_of ...
  ...  
end


class Feature < ActiveRecord::Base
  attr_accessible ...

  validates_presence_of ...
  validates_uniqueness_of ...

  has_many :listing_features
  has_many :listings, :through => :listing_features
end


class ListingFeature < ActiveRecord::Base
  attr_accessible :feature_id, :listing_id

  belongs_to :feature  
  belongs_to :listing
end

I'm using Rails 3.1.rc4, FactoryGirl 2.0.2, factory_girl_rails 1.1.0, and rspec. Here is my basic rspec rspec sanity check for the :listing factory:

it "creates a valid listing from factory" do
  Factory(:listing).should be_valid
end

Here is Factory(:listing)

FactoryGirl.define do
  factory :listing do
    headline    'headline'
    home_desc   'this is the home description'
    association :user, :factory => :user
    association :layout, :factory => :layout
    association :features, :factory => :feature
  end
end

The :listing_feature and :feature factories are similarly setup.
If the association :features line is commented out, then all my tests pass.
When it is

association :features, :factory => :feature

the error message is
undefined method 'each' for #<Feature> which I thought made sense to me because because listing.features returns an array. So I changed it to

association :features, [:factory => :feature]

and the error I get now is ArgumentError: Not registered: features Is it just not sensible to be generating factory objects this way, or what am I missing? Thanks very much for any and all input!