added smile model

app/models/answer.rb

@@ -1,5 +1,6 @@
 class Answer < ActiveRecord::Base
   belongs_to :user
   belongs_to :question
-  has_many :comments
+  has_many :comments, dependent: :destroy
+  has_many :smiles, dependent: :destroy
 end

app/models/smile.rb

@@ -0,0 +1,6 @@
+class Smile < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :answer
+  validates :user_id, presence: true
+  validates :answer_id, presence: true
+end

app/models/user.rb

@@ -70,4 +70,20 @@ class User < ActiveRecord::Base
   def following?(target_user)
     friends.include? target_user
   end
+
+  # smiles an answer
+  # @param answer [Answer] the answer to smile
+  def smile(answer)
+    Smile.create(user: self, answer: answer)
+    increment! :smiled_count
+    answer.increment! :smile_count
+  end
+
+  # unsmile an answer
+  # @param answer [Answer] the answer to unsmile
+  def unsmile(answer)
+    Smile.find_by(user: self, answer: answer).destroy
+    decrement! :smiled_count
+    answer.decrement! :smile_count
+  end
 end

db/migrate/20141130175749_create_smiles.rb

@@ -0,0 +1,14 @@
+class CreateSmiles < ActiveRecord::Migration
+  def change
+    create_table :smiles do |t|
+      t.integer :user_id
+      t.integer :answer_id
+
+      t.timestamps
+    end
+
+    add_index :smiles, :user_id
+    add_index :smiles, :answer_id
+    add_index :smiles, [:user_id, :answer_id], unique: true
+  end
+end

db/migrate/20141130180152_rename_columns_in_answers.rb

@@ -0,0 +1,7 @@
+class RenameColumnsInAnswers < ActiveRecord::Migration
+  def change
+    rename_column :answers, :comments, :comment_count
+    remove_column :answers, :likes
+    change_column :answers, :comment_count, :integer, default: 0, null: false
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20141130130221) do
+ActiveRecord::Schema.define(version: 20141130180152) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -19,8 +19,7 @@ ActiveRecord::Schema.define(version: 20141130130221) do
   create_table "answers", force: true do |t|
     t.string   "content"
     t.integer  "question_id"
-    t.integer  "comments"
-    t.integer  "likes"
+    t.integer  "comment_count", default: 0, null: false
     t.integer  "user_id"
     t.datetime "created_at"
     t.datetime "updated_at"
@@ -70,6 +69,17 @@ ActiveRecord::Schema.define(version: 20141130130221) do
   add_index "relationships", ["source_id"], name: "index_relationships_on_source_id", using: :btree
   add_index "relationships", ["target_id"], name: "index_relationships_on_target_id", using: :btree
 
+  create_table "smiles", force: true do |t|
+    t.integer  "user_id"
+    t.integer  "answer_id"
+    t.datetime "created_at"
+    t.datetime "updated_at"
+  end
+
+  add_index "smiles", ["answer_id"], name: "index_smiles_on_answer_id", using: :btree
+  add_index "smiles", ["user_id", "answer_id"], name: "index_smiles_on_user_id_and_answer_id", unique: true, using: :btree
+  add_index "smiles", ["user_id"], name: "index_smiles_on_user_id", using: :btree
+
   create_table "users", force: true do |t|
     t.string   "email",                  default: "",    null: false
     t.string   "encrypted_password",     default: "",    null: false