In part 3 we filled our database with a dozen of Ruby on Rails books with details taken from Amazon Web Services (AWS). We also fetched the book image and stored it into our own servers. In this part, we'll go back to AWS and fetch the book sales.

Book Sale Model

We need a model to store the sales rank we fetch from Amazon, so let's generate that now:

bundle exec rails generate model book_sale book:belongs_to sales_rank:integer

For each book we save the sales rank on the day we fetch it from Amazon. Let's do that next.

Amazon Proxy

In part 3 we created an AmazonProxy class to communicate with AWS. We'll add a few methods to that class to get the sales rank information.

# app/models/amazon_proxy.rb

def self.import_sale(asin)
  item = fetch(asin)
  save_sale asin, item


  def self.save_sale(asin, item)
    book = Book.find_by(asin: asin)
    existing = book.book_sales.where('DATE(created_at) = ?', Time.current.to_date).first
    if existing.nil?
      sales_rank = item.get('SalesRank')
      book.book_sales.create(sales_rank: sales_rank)

First we use our fetch method we created on the previous post to grab the book details by the book's unique identifier (ASIN). Then (in the save_sale method) we check that we don't have a ranking for the book on the same day. If we don't, we grab the SalesRank item attribute from AWS and create a new sale record for the book in the data store. At the end, we update the sales movement of the book. For that we create an instance method on the Book model:

def update_sales_rank_movement
  recent_sales = sales.last(2)
  return if recent_sales.size == 0
  options = { previous_sales_rank: 0, last_sales_rank: recent_sales[0].sales_rank }
  options[:previous_sales_rank] = recent_sales[1].sales_rank if recent_sales.size > 1
  update options

This method is simple: we get the last two sales for the book and update the previous_sales_rank and 'lastsalesrank' with the appropriate record. These two fields will then be used by the sales_rank_movement method:

def sales_rank_movement
  @sales_rank_movement ||= (last_sales_rank || 0) - (previous_sales_rank || 0)

As we see above, the movement shows the difference between the last sale and the one prior to that.


Next, let's create the Rake task that will populate our database everyday.

desc 'Import sales ranks'
task :sales => :environment do
  Book.all.each do |book|
    AmazonProxy.import_sale book.asin

No much mystery here, we navigate through the books and call the AmazonProxy method to import the book's sale.


In this part of How to create a Rails app series, we imported the daily book sales from Amazon and updated our book records. This allows us to show the sales movements daily. In the next part, we'll build the page that shows the books and their sales ranking.

You can check out the previous posts in this series below:

Books on Rails live example.