Update post votes after save post_vote

app/models/post_vote.rb

@@ -4,4 +4,12 @@ class PostVote < ActiveRecord::Base
   enum value: { down: -1, up: 1 }
 
   validates :value, presence: true
+
+  after_save :update_post_votes
+
+  def update_post_votes
+    if value_changed?
+      post.increment! :votes, VALUE[value]
+    end
+  end
 end

db/migrate/20140104125105_create_topics.rb

@@ -1,8 +1,8 @@
 class CreateTopics < ActiveRecord::Migration
   def change
     create_table :topics do |t|
-      t.string :title
       t.references :user, index: true
+      t.string :title
 
       t.timestamps
     end

db/migrate/20140104130551_create_posts.rb

@@ -1,10 +1,11 @@
 class CreatePosts < ActiveRecord::Migration
   def change
     create_table :posts do |t|
-      t.text :content
-      t.integer :post_number
       t.references :topic, index: true
       t.references :user, index: true
+      t.text :content
+      t.integer :post_number
+      t.integer :votes, default: 0
 
       t.timestamps
     end

db/schema.rb

@@ -29,10 +29,11 @@ ActiveRecord::Schema.define(version: 20140114124558) do
   add_index "post_votes", ["user_id"], name: "index_post_votes_on_user_id", using: :btree
 
   create_table "posts", force: true do |t|
-    t.text     "content"
-    t.integer  "post_number"
     t.integer  "topic_id"
     t.integer  "user_id"
+    t.text     "content"
+    t.integer  "post_number"
+    t.integer  "votes",       default: 0
     t.datetime "created_at"
     t.datetime "updated_at"
   end
@@ -41,8 +42,8 @@ ActiveRecord::Schema.define(version: 20140114124558) do
   add_index "posts", ["user_id"], name: "index_posts_on_user_id", using: :btree
 
   create_table "topics", force: true do |t|
-    t.string   "title"
     t.integer  "user_id"
+    t.string   "title"
     t.datetime "created_at"
     t.datetime "updated_at"
   end

test/models/post_vote_test.rb

@@ -7,4 +7,25 @@ class PostVoteTest < ActiveSupport::TestCase
     assert_equal 'up', vote.value
     assert vote.up?
   end
+
+  test "should inc post votes" do
+    post = create(:post)
+    assert_equal 0, post.votes
+
+    create(:post_vote, post: post, value: 'up')
+    assert_equal 1, post.votes
+
+    create(:post_vote, post: post, value: 'down')
+    assert_equal 0, post.votes
+  end
+
+  test "should inc post votes after vote update" do
+    post = create(:post)
+
+    vote = create(:post_vote, post: post, value: 'up')
+    assert_equal 1, post.votes
+
+    vote.update_attribute :value, 'down'
+    assert_equal 0, post.votes
+  end
 end