Ruby on Rails fixtures not loading relations

I have two models, Event and Registration:

class Event < ApplicationRecord
  has_many :registrations
end

class Registration < ApplicationRecord
  belongs_to :event
end

I also have fixtures for these models:

event_one:
  [...]

registration_one:
  event: event_one

However, in test, if I get the event and check its registrations, there are none.

event = events(:event_one)
puts event.registrations.count # Prints: 0

How do I make sure these associations are loaded for tests?