followable with redis Sorted-Set

Gemfile

# Gemfile
gem 'redis', '~> 3.0.1'

redis.rb

# config/initializers/redis.rb
$redis = Redis.new(:host => 'localhost', :port => 6379)

user.rb

# app/models/user.rb

class User

  def follow(user)
    score = Time.now.to_i

    $redis.multi do
      $redis.zadd(friendship_redis_key(:following), score, followed.id)
      $redis.zadd(followed.friendship_redis_key(:followers), score, id)
    end
  end

  def unfollow(user)
    $redis.multi do
      $redis.zrem(friendship_redis_key(:following), user.id)
      $redis.zrem(user.friendship_redis_key(:followers), id)
    end
  end

  def followers
    User.where(:id => follower_ids)
  end

  def following
    User.where(:id => following_ids)
  end

  def follower_ids
    $redis.zrevrange(friendship_redis_key(:followers), 0, -1)
  end

  def following_ids
    $redis.zrevrange(friendship_redis_key(:following), 0, -1)
  end

  def following?(user)
    !$redis.zscore(friendship_redis_key(:following), user.id).nil?
  end

  def followed_by?(user)
    !$redis.zscore(friendship_redis_key(:followers), user.id).nil?
  end

  def followers_count
    $redis.zcard(friendship_redis_key(:followers))
  end

  def following_count
    $redis.zcard(friendship_redis_key(:following))
  end

  private
    # generate redis keys
    def friendship_redis_key(str)
      "user:#{id}:#{str}"
    end
end