Remove duplication in has_one associations

Seven `has_one` associations were defined in the same way, so the
code can be simplified by declaring `has_one` programmatically.

This commit does not change any functionality, with two exceptions:
"account.channel" and "video.rating" were the only two `has_one`
associations that would return +nil+ when the association was not
found, rather than raising an Error. Now they raise an error too,
in coherence with the other associations.

lib/yt/associations.rb

@@ -33,6 +33,24 @@ def has_many(attributes, options = {})
       delegate *options[:delegate], to: attributes if options[:delegate]
     end
 
-    alias has_one has_many
+
+    def has_one(attribute, options = {})
+      delegate *options[:delegate], to: attribute if options[:delegate]
+
+      attributes = attribute.to_s.pluralize
+      require "yt/collections/#{attributes}"
+      mod = attributes.sub(/.*\./, '').camelize
+      collection = "Yt::Collections::#{mod.pluralize}".constantize
+
+      define_method attribute do
+        ivar = instance_variable_get "@#{attribute}"
+        instance_variable_set "@#{attribute}", ivar || send(attributes).first!
+      end
+
+      define_method attributes do
+        ivar = instance_variable_get "@#{attributes}"
+        instance_variable_set "@#{attributes}", ivar || collection.of(self)
+      end
+    end
   end
 end

lib/yt/models/video.rb

@@ -6,6 +6,25 @@ class Video < Resource
       has_one :details_set, delegate: [:duration]
       has_one :rating
       has_many :annotations
+
+      def liked?
+        rating.rating == :like
+      end
+
+      def like
+        rating.update :like
+        liked?
+      end
+
+      def dislike
+        rating.update :dislike
+        !liked?
+      end
+
+      def unlike
+        rating.update :none
+        !liked?
+      end
     end
   end
 end

spec/associations/device_auth/account_spec.rb

@@ -0,0 +1,7 @@
+require 'spec_helper'
+require 'yt/models/account'
+
+describe Yt::Account, :device_app do
+  it { expect($account.channel).to be_a Yt::Channel }
+  it { expect($account.user_info).to be_a Yt::UserInfo }
+end

spec/associations/device_auth/channel_spec.rb

@@ -0,0 +1,20 @@
+require 'spec_helper'
+require 'yt/models/channel'
+
+describe Yt::Channel, :device_app do
+  let(:channel) { Yt::Channel.new id: id, auth: $account }
+
+  context 'given an existing channel' do
+    let(:id) { 'UCxO1tY8h1AhOz0T4ENwmpow' }
+
+    it { expect(channel.snippet).to be_a Yt::Snippet }
+    it { expect(channel.status).to be_a Yt::Status }
+  end
+
+  context 'given an unknown channel' do
+    let(:id) { 'not-a-channel-id' }
+
+    it { expect{channel.snippet}.to raise_error Yt::Errors::NoItems }
+    it { expect{channel.status}.to raise_error Yt::Errors::NoItems }
+  end
+end

spec/associations/device_auth/playlist_spec.rb

@@ -0,0 +1,20 @@
+require 'spec_helper'
+require 'yt/models/playlist'
+
+describe Yt::Playlist, :device_app do
+  let(:playlist) { Yt::Playlist.new id: id, auth: $account }
+
+  context 'given an existing playlist' do
+    let(:id) { 'PLSWYkYzOrPMRCK6j0UgryI8E0NHhoVdRc' }
+
+    it { expect(playlist.snippet).to be_a Yt::Snippet }
+    it { expect(playlist.status).to be_a Yt::Status }
+  end
+
+  context 'given an unknown playlist' do
+    let(:id) { 'not-a-playlist-id' }
+
+    it { expect{playlist.snippet}.to raise_error Yt::Errors::NoItems }
+    it { expect{playlist.status}.to raise_error Yt::Errors::NoItems }
+  end
+end

spec/associations/device_auth/ids_spec.rb → spec/associations/device_auth/resource_spec.rb

@@ -1,7 +1,7 @@
 require 'spec_helper'
-require 'yt/associations/ids'
+require 'yt/models/resource'
 
-describe Yt::Associations::Ids, :device_app do
+describe Yt::Resource, :device_app do
   subject(:resource) { Yt::Resource.new url: url, auth: $account }
 
   describe '#id' do