added some comments to models and controllers

app/controllers/customers_controller.rb

@@ -1,4 +1,5 @@
 class CustomersController < ApplicationController
+# this action returns information for all of the customers in the api request
 def index
   customers = Customer.all
   # render json: customers, status: :ok

app/controllers/movies_controller.rb

@@ -1,5 +1,6 @@
 class MoviesController < ApplicationController
 
+# this action will return a list of all the movies in the api response
   def index
     movies = Movie.all
     render(
@@ -8,6 +9,7 @@ def index
     )
   end
 
+# this action will return information for a sinlge instance of Movie in the api request
   def show
     movie = Movie.find_by(id: params[:id])
 
@@ -25,6 +27,7 @@ def show
 
   end
 
+# this action allows a new instance of Movie to be created by the user of the api
   def create
     movie = Movie.new(movie_params)
 

app/controllers/rentals_controller.rb

@@ -1,19 +1,24 @@
 class RentalsController < ApplicationController
 
+  # This method is used to 'check-out' a movie.
+  # A new instance of Rental is created using a customer_id and movie_id passed in by the user
+  # This instance of Rental's checked_out status defaults to true
   def create
     rental = Rental.new(rental_data)
     rental.due_date = Date.today + 3
     if rental.save
       render(
         json: {id: rental.id, customer_id: rental.customer.id, movie_id: rental.movie.id, due_date: rental.due_date}
       )
+    # if the user did not pass in valid information (i.e. there was no customer_id or movie_id) then the new instance of Rental will not save
     else
       render(
         json: {error: rental.errors.messages}, status: :bad_request
       )
     end # if/else
   end # create
 
+  # the update method is used to 'check-in' a movie. It changes the checked_out attribute for the rental to false
   def update
     rental = Rental.find_by(id: params[:id])
     if rental
@@ -27,6 +32,7 @@ def update
           json: {error: rental.errors.messages}, status: :bad_request
         )
       end # if/else
+    # if the rental_id passed in does not exist then not_found is returned
     else
       render(
         json: {"no rental found": true}, status: :not_found

app/models/customer.rb

@@ -5,6 +5,8 @@ class Customer < ApplicationRecord
 
   validates :name, presence: true
 
+  # this method calculates the number of movies checked out for the index action for customer
+  # when we render json in the index action it knows to run this method for each instance of Customer (treating the each instance as self) without having to pass any parameters to this method.
   def movies_checked_out_count
     self.rentals.where(checked_out: true).count
   end # movies_checked_out

app/models/movie.rb

@@ -3,6 +3,8 @@ class Movie < ApplicationRecord
 
   validates :title, :inventory, presence: true
 
+  # this method shows the availible inventory for a specific movies_path
+  # when this method is called in the show action for movie (:available_inventory) it knows that self is the instance of movie that 'movie' points to without having to pass any parameters to this method
   def available_inventory
     (self.inventory) - (self.rentals.where(checked_out: true).count)
   end