Add opponent association

lib/chess/auth/user.ex

@@ -11,6 +11,7 @@ defmodule Chess.Auth.User do
     field :username, :string
 
     has_many :games, Chess.Store.Game
+    has_many :other_games, Chess.Store.Game, foreign_key: :opponent_id
 
     timestamps()
   end

lib/chess/store/game.ex

@@ -13,6 +13,7 @@ defmodule Chess.Store.Game do
     field :board, :map
 
     belongs_to :user, Chess.Auth.User
+    belongs_to :opponent, Chess.Auth.User, references: :id
 
     timestamps()
   end

priv/repo/migrations/20180218165621_add_opponent_to_game.exs

@@ -0,0 +1,9 @@
+defmodule Chess.Repo.Migrations.AddOpponentToGame do
+  use Ecto.Migration
+
+  def change do
+    alter table("games") do
+      add :opponent_id, references(:users)
+    end
+  end
+end