user.rb

class User < ApplicationRecord
  has_many :posts, dependent: :destroy
  has_many :connections, dependent: :destroy

  has_many :follower_connections, foreign_key: :following_id, class_name: 'Connection'
  has_many :followers, through: :follower_connections, source: :follower

  has_many :following_connections, foreign_key: :follower_id, class_name: 'Connection'
  has_many :following, through: :following_connections, source: :following

  has_many :following_posts, through: :following, source: :post

  def fetch_feed
    cache_keys = redis.zrevrange(cache_key, 0, -1)
                      .map { |id| Post.cache_key_for(id: id) }

    return [] unless cache_keys.any?

    Rails.cache.fetch_multi(*cache_keys, expires: 1.minute, race_condition_ttl: 5.seconds) do |key|
      id = Post.id_from_cache_key(key: key)
      Post.find_by(id: id)
    end.values
  end
  
  def fetch_read_feed
    Post.eager_load(:user)
        .where(user: self.following + [self])
        .order('posts.created_at DESC')
  end

  def add_to_cache(post:)
    redis.zadd(cache_key, post.created_at.to_i, post.id)
  end
end

user = User.first
# => User Load (0.3ms)  SELECT  `users`.* FROM `users` ORDER BY `users`.`id` ASC LIMIT 1
user.followers
# => SELECT  `users`.* FROM `users` INNER JOIN `connections` ON `users`.`id` = `connections`.`follower_id` WHERE `connections`.`following_id` = 4
user.following
# => SELECT  `users`.* FROM `users` INNER JOIN `connections` ON `users`.`id` = `connections`.`following_id` WHERE `connections`.`follower_id` = 4

user.fetch_feed
# => User Load (0.4ms)  SELECT `users`.* FROM `users` INNER JOIN `connections` ON `users`.`id` = `connections`.`following_id` WHERE `connections`.`follower_id` = 4
# => SQL (0.7ms)  SELECT  ..... FROM `posts` LEFT OUTER JOIN `users` ON `users`.`id` = `posts`.`user_id` WHERE `posts`.`user_id` IN (5, 4) ORDER BY posts.created_at DESC LIMIT 11



user.fetch_feed
# => Post Load (5.9ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 21 LIMIT 1
# => Post Load (0.7ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 20 LIMIT 1
# => Post Load (0.6ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 19 LIMIT 1
# => Post Load (0.3ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 18 LIMIT 1
# => Post Load (0.3ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 17 LIMIT 1
# => Post Load (0.5ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 16 LIMIT 1
# => Post Load (0.4ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 15 LIMIT 1
# => Post Load (0.3ms)  SELECT  `posts`.* FROM `posts` WHERE `posts`.`id` = 14 LIMIT 1