Query optimization !

counter_cache.rb

# Initial: No counter_cach for tweet's favorites, doing a_tweet.favorites.size pulls all favorite records..
class Favorite < ActiveRecord::Base
  belongs_to :tweet
end


# Final: Setting a counter_cache to keeptrack of favorites count
class AddCounterCacheColumnToTweets < ActiveRecord::Migration
  def self.up
    add_column  :tweets,
                :favorites_count,
                :integer,
                :default => 0
  end
  def self.down
    remove_column :tweets, :favorites_count
  end
end

class Favorite < ActiveRecord::Base
  belongs_to :tweet,
             :counter_cache => :favorites_count
end

# => a_tweet.favorites.size 
# USE SIZE !!! not length or count

find_each.rb

# Use find_each : not to load all records once (especially with big DB)
Tweet.find_each( :batch_size => 5 ) do |tweet|
  puts tweet.status
end

index.rb

# Adding index that will optimize current_user.followings.order('created_at desc')
class AddIndexesToFollowings < ActiveRecord::Migration
  def self.up
    add_index :followings, [:user_id, :created_at]
  end

  def self.down
    remove_index :followings, [:user_id, :created_at]
  end
end

# => ALWAYS INDEX FOREIGN KEY 

N+1.rb

# Initial
# N+1 query : 1. getting the tweet's favorites array
#             2. for N favorites, get the associated user 
class Tweet < ActiveRecord::Base
  has_many :favorites

  def favorited_users
    self.favorites.collect {|fav| fav.user }
  end
end

# Initial
# 2 queries : 1. getting the tweet's favorites array including their users' id
#             2. getting the users corresponding to thes id array (1 query)
class Tweet < ActiveRecord::Base
  has_many :favorites

  def favorited_users
    self.favorites.includes(:user).collect {|fav| fav.user }
  end
end