ActiveRecord caching has_many :through

The second assert of this test incorrectly fails. However, when the first assert is removed, the second assert will pass. Looks like ActiveRecord is caching the value of me.favorite_pets, and Favorite.new isn't invalidating the cache.

test "favorite pet" do
  me = User.new({:name => 'Me'})
  sparky = Pet.new({:name => 'Sparky'})
  assert_equal [], me.favorite_pets
  Favorite.new({:user => me, :favorite_thing => sparky}).save
  assert_equal [sparky], me.favorite_pets
end

The models are as follows:

class User < ActiveRecord::Base
  has_many :favorites
  has_many :favorite_pets, :through => :favorites, :source => :pet
  has_many :favorite_movies, :through => :favorites, :source => :movie
end

class Favorite < ActiveRecord::Base
  belongs_to :user
  belongs_to :favorite_thing, :polymorphic => true
  belongs_to :pet, :foreign_key => :favorite_thing_id
  belongs_to :movie, :foreign_key => :favorite_thing_id
end