The Game object is decorated before being rendered

I'd like to let the rendered object (`@game) to tells us how it wants to
be rendered, instead of letting the controller decide (ie. leaving the
default call to `render` in the controller action, and calling `render @game`
from the view). To do so, we have to override the default method
`to_partial_path` from `ActiveModel::Conversion`.

However, the choice of the relevant partial depends on both the game's
current state (is it running, over, etc.) **and** the current context,
namely, _who_ is looking at the game: is it one of the current players,
a new player waiting to join the game, or a guest.

Since we don't want the `Game` model to be context-aware, we need a new object
to handle this responsibility; something that would represent a game _as seen
by a certain person_. This looks like a good role for a _decorator_.

I chose to implement this decorator with the simplest object possible: a
`SimpleDelegator`. Since we're following Rails' convention of naming objects
(expect for models) after their _nature_ (ie. `foo_controller`, `foo_helper`,
etc.), I named this object `GameExhibit`.

(I chose the `Exhibit` suffix after Avdi Grimm's
[Objects on Rails](http://objectsonrails.com/#ID-2656c30c-080a-4a4e-a53e-4fbaad39c262),
where Avdi makes a good case against using more familiar terms such as `Decorator`
or `Presenter`.)

A note on the specs: I tried to follow the apparent convention of large contexts
and minimal expectations. I'm not entirely convinced (it looks a bit too cryptic
and magical to me), so feedback is especially welcome here.

Author: r3trofitted

app/exhibits/game_exhibit.rb

@@ -0,0 +1,31 @@
+class GameExhibit < SimpleDelegator
+  def initialize(game, current_player)
+    @current_player = current_player
+    super(game)
+  end
+  
+  def to_partial_path
+    case state
+    when "finished"
+      "games/finished_game"
+    when "awaiting_opponent"
+      player_in_game? ? super : "games/awaiting_opponent_game"
+    when "running"
+      player_in_game? ? super : "games/full_game"
+    end
+  end
+  
+  def to_model
+    __getobj__
+  end
+  
+  def class
+    __getobj__.class
+  end
+  
+  private
+  
+  def player_in_game?
+    @current_player && @current_player.game == __getobj__
+  end
+end

spec/exhibits/game_exhibit_spec.rb

@@ -0,0 +1,49 @@
+require "rails_helper"
+
+RSpec.describe "GameExhibit" do
+  let(:game) { Game.new }
+  let(:player) { Player.new }
+  subject(:game_exhibit) { GameExhibit.new(game, player) }
+  
+  describe "#to_partial_path" do
+    subject { game_exhibit.to_partial_path }
+    
+    context "when the current player is actually playing the game" do
+      before { player.game = game }
+      
+      context "if the game is waiting for an opponent" do
+        before { allow(game).to receive(:state).and_return("awaiting_opponent") }
+        it { is_expected.to eql("games/game") }
+      end
+      
+      context "if the game is running" do
+        before { allow(game).to receive(:state).and_return("running") }
+        it { is_expected.to eql("games/game") }
+      end
+      
+      context "if the game is over" do
+        before { allow(game).to receive(:state).and_return("finished") }
+        it { is_expected.to eql("games/finished_game") }
+      end
+    end
+    
+    context "when the current player is not part of the game" do
+      before { player.game = Game.new }
+      
+      context "if the game is waiting for an opponent" do
+        before { allow(game).to receive(:state).and_return("awaiting_opponent") }
+        it { is_expected.to eql("games/awaiting_opponent_game") }
+      end
+      
+      context "if the game is running" do
+        before { allow(game).to receive(:state).and_return("running") }
+        it { is_expected.to eql("games/full_game") }
+      end
+      
+      context "if the game is over" do
+        before { allow(game).to receive(:state).and_return("finished") }
+        it { is_expected.to eql("games/finished_game") }
+      end
+    end
+  end
+end