20201111082153_create_restaurants.rb

class CreateRestaurants < ActiveRecord::Migration[6.0]
  def change
    create_table :restaurants do |t|
      t.string :name
      t.string :address
      t.integer :stars

      t.timestamps
    end
  end
end

20201111083213_create_reviews.rb

class CreateReviews < ActiveRecord::Migration[6.0]
  def change
    create_table :reviews do |t|
      t.text :content
      t.references :restaurant, null: false, foreign_key: true

      t.timestamps
    end
  end
end

edit.html.erb

<h1>Editing Restaurant</h1>

<%= render 'form', restaurant: @restaurant %>

<%= link_to 'Show', @restaurant %> |
<%= link_to 'Back', restaurants_path %>

new.html.erb

<h1>Add review to a <%= @restaurant.name %> </h1>


 <!-- POST   /restaurants/:restaurant_id/reviews -->
<%= simple_form_for([@restaurant, @review]) do |f| %>
  <%= f.input :content %>
  <%= f.submit class: "btn btn-secondary" %>
<% end %>

restaurant.rb

class Restaurant < ApplicationRecord
  has_many :reviews
  # Allows you to do:
  # resto = Restaurant.find(1)
  # resto.reviews => Returns array of Review
end

restaurants_controller.rb

class RestaurantsController < ApplicationController
  before_action :set_restaurant, only: [:show, :edit, :update, :destroy]

  # GET /restaurants
  def index
    @restaurants = Restaurant.all
  end

  # GET /restaurants/1
  def show
  end

  # GET /restaurants/new
  def new
    @restaurant = Restaurant.new
  end

  # GET /restaurants/1/edit
  def edit
  end

  # POST /restaurants
  def create
    @restaurant = Restaurant.new(restaurant_params)

    if @restaurant.save
      redirect_to @restaurant, notice: 'Restaurant was successfully created.'
    else
      render :new
    end
  end

  # PATCH/PUT /restaurants/1
  def update
    if @restaurant.update(restaurant_params)
      redirect_to @restaurant, notice: 'Restaurant was successfully updated.'
    else
      render :edit
    end
  end

  # DELETE /restaurants/1
  def destroy
    @restaurant.destroy
    redirect_to restaurants_url, notice: 'Restaurant was successfully destroyed.'
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_restaurant
      @restaurant = Restaurant.find(params[:id])
    end

    # Only allow a trusted parameter "white list" through.
    def restaurant_params
      params.require(:restaurant).permit(:name, :address, :stars)
    end
end

review.rb

class Review < ApplicationRecord
  belongs_to :restaurant
  # Allows you to:
  # rev = Review.find(1)
  # rev.restaurant => Returns parent Restaurant
  validates :content, presence: { message: "NO YOU CANT" }
end

reviews_controller.rb

class ReviewsController < ApplicationController
  def new
    @restaurant = Restaurant.find(params[:restaurant_id])
    @review = Review.new
  end

  # POST /restaurants/3/reviews
  # Body contains data for attributes
  def create
    @restaurant = Restaurant.find(params[:restaurant_id])
    @review = Review.new(review_params)
    @review.restaurant = @restaurant
    if @review.save # No guarantee it will save!
      # Happy path :)
      # Don't redirect if save went wrong
      redirect_to restaurant_path(@restaurant)
    else
      # Sad path :(
      render "new"
    end
  end

  def destroy
    @review = Review.find(params[:id])
    @review.destroy
    redirect_to restaurant_path(@review.restaurant)
  end

  private

  def review_params
    params.require(:review).permit(:content)
  end
end

routes.rb

Rails.application.routes.draw do
  resources :restaurants do
    # Nested
    resources :reviews, only: [:new, :create]
  end
  # Not nested
  resources :reviews, only: :destroy

  # get "restaurants/:restaurant_id/reviews/new", to: "reviews#new"
end

show.html.erb

<p id="notice"><%= notice %></p>

<p>
  <strong>Name:</strong>
  <%= @restaurant.name %>
</p>

<p>
  <strong>Address:</strong>
  <%= @restaurant.address %>
</p>

<p>
  <strong>Stars:</strong>
  <%= @restaurant.stars %>
</p>

<div>
  <ul>
    <% @restaurant.reviews.each do |review| %>
      <li>
        <%= review.content %> | <%= link_to "❌", review_path(review), method: :delete  %>
      </li>
    <% end %>
  </ul>
</div>

<%= link_to 'Edit', edit_restaurant_path(@restaurant) %> |
<%= link_to 'Back', restaurants_path %> |
<%= link_to 'Leave a review', new_restaurant_review_path(@restaurant) %>