FactoryGirl set up Mongoid associations

FactoryGirl set up Mongoid associations

I have 2 Mongoid models:
One is Activity, which has belongs_to :activity_type.
Another is ActivityType, which has has_many :activities.
In FactoryGirl, I'm trying to create Activities for each ActivityType, the
following code doesn't work, any suggestions?
factory :activity_type_watch_video, class: ActivityType do
name 'Watch a video'
end
factory :activity_type_talk_someone, class: ActivityType do
name 'Talk to someone'
end
factory :activity do
sequence(:name) {|n| "Test Activity #{n}" }
association :activity_type_watch_video
end