Convert API Response Into a Model in Rails

Let’s get down straight to the business.

Let’s take an example of showing the invoices to the user from Stripe.

First, let’s write what we would like to have without worrying about the implementation.

I like short and clean controllers. Something like the following would do.

app/controllers/invoices_controller.rb

class InvoicesController < ApplicationController
  def index
    @invoices = Invoice.find_all_by_user(current_user)
  end

  def show
    @invoice = Invoice.new(params[:id])
    render
  end
end

Below is what I would like in my view: app/views/invoices/_invoice.html.erb

<% unless (invoice.total == 0) %>
  <tr>
    <td><%= link_to invoice.number, invoice_path(invoice.id) %></td>
    <td><%= invoice.date %></td>
    <td><%= number_to_currency(invoice.total, negative_format: "(%u%n)") %></td>
    <td><%= invoice.period_start %> to <%= invoice.period_end %></td>
    <td><%= invoice.paid? ? 'Paid' : 'Unpaid' %></td>
  </tr>
<% end %>

I have experienced that writing the interface first, as we did above, gives me a lot of clarity during implementation. Now, let’s start implementing it in a model.

app/models/invoice.rb

class Invoice

  attr_reader :stripe_invoice

  def self.find_all_by_user(user)
    if user.present?
      stripe_invoices_for_user(user).map do |invoice|
        new(invoice)
      end
    else
      []
    end
  end

  def initialize(invoice_id_or_object)
    if invoice_id_or_object.is_a? String
      @stripe_invoice = retrieve(invoice_id_or_object)
    else
      @stripe_invoice = invoice_id_or_object
    end
  end

  def to_partial_path
    "invoices/#{self.class.name.underscore}"
  end

  def id
    stripe_invoice.id
  end

  def number
    stripe_invoice.number
  end

  def total
    cents_to_dollars(stripe_invoice.total)
  end

  def date
    convert_stripe_time(stripe_invoice.date)
  end

  def paid?
    stripe_invoice.paid
  end

  def subscription
    stripe_invoice.subscription
  end

  def period_start
    convert_stripe_time(stripe_invoice.period_start)
  end

  def period_end
    convert_stripe_time(stripe_invoice.period_end)
  end

  def user
    @user ||= User.find_by(stripe_customer_id: stripe_invoice.customer)
  end

  def balance
    if paid?
      0.00
    else
      amount_due
    end
  end

  def amount_due
    cents_to_dollars(stripe_invoice.amount_due)
  end

  def subtotal
    cents_to_dollars(stripe_invoice.subtotal)
  end

  def amount_paid
    if paid?
      amount_due
    else
      0.00
    end
  end

  def plan
    stripe_invoice.lines.data[0].plan.name
  end

  def plan_amount
    cents_to_dollars stripe_invoice.lines.data[0].plan.amount
  end

  def pay
    stripe_invoice.pay
  end

  def self.pay_if_pending(user)
    invoices = find_all_by_user(user)
    unless invoices.empty? || invoices.first.paid?
      invoices.first.pay
    end
  end

  def self.upcoming(user)
    new(Stripe::Invoice.upcoming(customer: user.stripe_customer_id) || nil )
  end

  private

  def self.stripe_invoices_for_user(user)
    Stripe::Invoice.all(customer: user.stripe_customer_id).data
  end

  def retrieve(invoice_id)
    Stripe::Invoice.retrieve(invoice_id)
  end

  def convert_stripe_time(time)
    Time.zone.at(time).strftime('%D')
  end

  def cents_to_dollars(amount)
    amount / 100.0
  end
end

Here, the Stripe gem takes exposes many methods on the response. But we can take any vanilla JSON response any do the same.

Many years ago, I learned this pattern form Upcase. Since then I am always parsing API responses like this. Upcase is free now and definitely worth a look.

Download the code as gist.