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
  include ApplicationHelper

  def index
    render json: fetch_newsfeeds(params[:page] || 1, 10)
  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"
      },
    ]
  }
]

_comment.json.jbuilder

# /views/newsfeed/_comment.json.jbuilder
json.type comment.class.name
json.user format_user(comment.user)
json.content comment.content

_post.json.jbuilder

# /views/newsfeed/_post.json.jbuilder
json.type post.class.name
json.content post.content
json.user format_user(post.user)

json.comments post.comments, partial: 'comment', as: :comment

application_helper.rb

module ApplicationHelper
  def format_user user
    {type: user.class.name, name: user.name}
  end

  def fetch_newsfeeds(page, limit)
    @posts = Post.order(:created_at).paginate(page: page, per_page: limit)
    render_to_string(template: "/newsfeed/feed.json.jbuilder", locals: { posts: @posts}, format: :json)
  end
end

feed.json.jbuilder

# /views/newsfeed/feed.json.jbuilder
json.array! @posts, partial: 'post', as: :post