Updating an association with Elixir Ecto

gistfile1.txt

defmodule PhoenixTimeline.Game do
...
  schema "games" do
    belongs_to :winner, PhoenixTimeline.Player
    has_many :players, PhoenixTimeline.Player
    ...
  end
...
end

defmodule PhoenixTimeline.Player do
...
  schema "players" do
    ...
    has_one :victory, PhoenixTimeline.Game, foreign_key: :winner_id
    belongs_to :game, PhoenixTimeline.Game
  end
...
end

game = Repo.get Game, game_id
player = Repo.get Player, player_id
Repo.preload player, :victory       #necessary?
assoc = Ecto.build_assoc(player, :victory, Map.from_struct game)
changeset = Player.changeset player, Map.from_struct assoc
Repo.update changeset