1_user.rb

class User < ApplicationRecord
  has_many :posts
  has_many :comments
  #  id                   :integer          not null, primary key
  #  name                 :string(50)       default("")
end

2_post.rb

class Post < ApplicationRecord
  belongs_to :user
  has_many :comments

  #  id                   :integer          not null, primary key
  #  user_id              :integer
  #  content              :text             default("")
  #  created_at           :datetime
end

3_comment.rb

class Comment < ApplicationRecord
  belongs_to :user
  belongs_to :post

  #  id                   :integer          not null, primary key
  #  user_id              :integer
  #  post_id              :integer
  #  content              :text             default("")
  #  created_at           :datetime
end

4_newsfeed_controller.rb

class NewsfeedController < ApplicationController
  # JSON endpoint that returns an array of Post objects in order of
  # newest first, to oldest last. Each Post contains a User object
  # (the author of the Post), and an array of Comments. Each Comment
  # will also include the User object of the Comment's author.
  
  # TODO: Newsfeed endpoint here
  def index
    #  ** is to ensure the attribute serializer allows the nested relationships in the json like user <-> comment
    render json: recent, include: '**', status: '200 OK'
  end

  def posts
    # We use include to avoid n+1 queries
    @post = Post
                .includes(:user)
                .includes(comments: [:user]) 
  end

  def recent
    posts.order(created_at: :desc).limit(50)
  end
end

5_example.json

[
  {
    "type": "Post",
    "content": "First post",
    "user": {
      "type": "User",
      "name": "Luke"
    },
    "comments": [
      {
        "type": "Comment",
        "user": {
          "type": "User",
          "name": "Leia"
        },
        "content": "First comment"
      },
      {
        "type": "Comment",
        "user": {
          "type": "User",
          "name": "Han"
        },
        "content": "Second comment"
      },
    ]
  },
  {
    "type": "Post",
    "content": "Second post",
    "user": {
      "type": "User",
      "name": "Darth Vader"
    },
    "comments": [
      {
        "type": "Comment",
        "user": {
          "type": "User",
          "name": "Boba Fett"
        },
        "content": "Third comment"
      },
      {
        "type": "Comment",
        "user": {
          "type": "User",
          "name": "Jabba"
        },
        "content": "Fourth comment"
      },
    ]
  }
]

@ericktai

Can you make the queries efficient?

By using includes, we avoid the back and forth chatter from app to sql server by loading the ids we need from the one model and piping them into the next query.
The generated sql is

  Post Load (0.6ms)  SELECT  `posts`.* FROM `posts` ORDER BY `posts`.`created_at` DESC LIMIT 50
  User Load (0.6ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` IN (...)
  Comment Load (2.4ms)  SELECT `comments`.* FROM `comments` WHERE `comments`.`post_id` IN (...)
  User Load (1.2ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` IN (...)

What if we want to reuse the query in other tasks? Would you organize the code in a different way?

Instead of putting all this in the controller I would factor it out to the model by using scopes or model methods that way other parts of the app can use it. After all Don't Repeat Yourself :)