Add counter cache for links

app/models/card.rb

@@ -12,7 +12,7 @@ class Card < ActiveRecord::Base
   belongs_to :awaken_type
   belongs_to :dokkan_card, class_name: 'Card', foreign_key: :dokkan_id
 
-  has_and_belongs_to_many :links
+  has_and_belongs_to_many :links, counter_cache: true
 
   has_many :evidences
 

db/migrate/20151103193404_add_links_counter_cache_to_cards.rb

@@ -0,0 +1,14 @@
+class AddLinksCounterCacheToCards < ActiveRecord::Migration
+  def up
+    add_column :cards, :links_count, :integer, default: 0, null: false
+
+    Card.reset_column_information
+    Card.find_each do |card|
+      card.update_attribute(:links_count, card.links.count)
+    end
+  end
+
+  def down
+    remove_column :cards, :links_count
+  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: 20151020212132) do
+ActiveRecord::Schema.define(version: 20151103193404) do
 
   create_table "awaken_types", force: :cascade do |t|
     t.string   "name"
@@ -36,6 +36,7 @@ ActiveRecord::Schema.define(version: 20151020212132) do
     t.integer  "hp_stat_id"
     t.integer  "atk_stat_id"
     t.integer  "def_stat_id"
+    t.integer  "links_count",      default: 0,         null: false
   end
 
   add_index "cards", ["awaken_type_id"], name: "index_cards_on_awaken_type_id"