Rails fixture doesn't recognize a belongs_to relationship

I have 2 models :

class User < ApplicationRecord
  has_many :games, dependent: :nullify

class Game < ApplicationRecord
  belongs_to :user
  belongs_to :playlist, optional: true

In my fixtures, game only belongs_to users. My only fixtures are :

user.yml

michel:
  id: 1
  pseudo: michelchardou
  email: mich@chardou.com
  encrypted_password: User.digest('password')
  admin: false

game.yml

game1:
  user: michel
  token: 123456
  total_score: 0

My basic tests work for

  test "should get expected game fixture token" do
    assert_equal '123456', @game1.token
    assert_equal 0, @game1.total_score
  end

but the following raises an error :

  test "should get expected game fixture user" do
    assert_equal 'michel', @game1.user
  end

the error is :

Failure:
GameTest#test_should_get_expected_game_fixture_user [/Users/pierre/code/pierregarciafr/wadzatsong/test/models/game_test.rb:25]:
Expected: "michel"
  Actual: nil

Why ?