Stripe MRR Calculation

mrr.rb

require 'stripe'
require 'ostruct'

# modified from: https://gist.github.com/jacobpatton/a68d228bf2414852d862
#
# puts Stripe::Mrr.new(api_key: 'api_key').mrr
#
module Stripe
  class Mrr
    attr_reader :api_key

    def initialize(args = {})
      raise ArgumentError, ":api_key is a required argument" unless args[:api_key]

      @api_key = args[:api_key]
      Stripe.api_key = @api_key

      @charges = []
      @time = Time.now.to_i
    end

    def mrr
      subscriptions.inject(0) do |sum, subscription|
        amount, interval = subscription.amount, subscription.interval
        amount = amount / 12 if interval == 'year'

        sum + amount
      end
    end

    def charges
      fetch_charges if @charges.empty?
      @charges
    end

    def subscriptions
      charges.inject([]) do |collection, charge|
        puts charge.id
        next collection unless charge.paid
        next collection if charge.invoice.nil?

        puts charge.invoice
        invoice = Stripe::Invoice.retrieve(charge.invoice)
        subscription = invoice.lines.data.find {|l| l.type == 'subscription'}
        if subscription.nil?
          puts 'no subscription'
          next collection
        end
        next collection unless subscription.period.end > @time

        collection.concat([OpenStruct.new(amount: invoice.amount_due, interval: subscription.plan.interval)])
      end
    end

    private

    def fetch_charges(opts = {})
      opts = {limit: 100}.merge!(opts)

      collection = Stripe::Charge.all(opts)
      @charges.concat(collection.data)

      fetch_charges(starting_after: @charges.last.id) if collection.has_more
    end
  end
end

I did a puts invoice.lines.data right before the error and this is the output

#<Stripe::StripeObject:0x3fd8bec477ec> JSON: {
  "subscriptions": [
    {"id":"sub_4lBuQuiFO22JLs","object":"line_item","type":"subscription","livemode":true,"amount":4900,"currency":"usd","proration":false,"period":{"start":1415742863,"end":1418334863},"subscription":null,"quantity":1,"plan":{"id":"simple","interval":"month","name":"Simple","created":1390971289,"amount":4900,"currency":"usd","object":"plan","livemode":true,"interval_count":1,"metadata":{},"statement_description":null},"description":null,"metadata":{}}
  ]
}