For a SAAS business, monthly recurring revenue (MRR) is their bread and butter. This small class fetches subscription records from Stripe to calculate the MRR.

mrr.rb

require 'stripe'

# Find the MRR for a given Stripe account.
# 
# (To do this it simply finds the total amount for all active 
# subscriptions (in cents). For most SAAS accounts, this is 
# enough, though modification is necessary to account for 
# things like trials, discounts, pro-rated cancellations, etc.)
#
# {stripe_id: YOUR_STRIPE_SECRET_KEY} - A hash with the key "stripe_id" and the correct secret key
#
# Examples
#
#   Stripe::Mrr.new(api_key: YOUR_STRIPE_SECRET_KEY)
#   # => 1000
#
# Returns an integer representing the MRR (in cents)

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
      @customers = []
    end

    def mrr
      subscriptions.inject(0){|sum, subscription| sum + subscription.plan.amount}
    end

    def customers
      fetch_customers if @customers.empty?
      @customers
    end

    def subscriptions
      customers.inject([]) do |collection, customer| 
        collection.concat(customer.subscriptions.data)
      end
    end

    private

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

        collection = Stripe::Customer.all(opts)
        @customers.concat(collection.data)

        fetch_customers(starting_after: @customers.last.id) if collection.has_more
      end

  end
end

mrr_spec.rb

require 'mrr'
require 'stripe_mock'

describe Stripe::Mrr do

  before { StripeMock.start }
  after  { StripeMock.stop }

  it "should raise an error if initialized without an :api_key argument"  do
    msg = ":api_key is a required argument"
    expect { Stripe::Mrr.new }.to raise_error(ArgumentError, msg)
  end
  
  it "should correctly set the API key" do
    api_key = "some_api_key"
    mrr     = build_mrr(api_key)

    mrr.api_key.should eq api_key
  end

  describe "#customers" do
    it "should return an array of customers" do
      mrr                     = build_mrr
      customer_1, customer_2  = Array.new(2, new_customer)

      # we need two pages of results
      # the Stripe API will respond with ListObjects, which we're mocking
      customer_list_page_1    = customer_list(true, [customer_1]) 
      customer_list_page_2    = customer_list(false, [customer_2]) 

      # the first page of results (has_more = true)
      allow(Stripe::Customer).to receive(:all).with(limit: 100) do
        customer_list_page_1
      end

      # the second page of results (has_more = false)
      allow(Stripe::Customer).to receive(:all).with(limit: 100, starting_after: customer_1.id) do
        customer_list_page_2
      end

      mrr.customers.should eq([customer_1, customer_2])
    end
  end

  describe "#subscriptions" do
    it "should return an array of subscriptions" do
      mrr               = build_mrr
      subscription      = double("subscription")
      subscription_list = double("list", data: [subscription])
      customer          = double("customer", subscriptions: subscription_list)

      allow(mrr).to receive(:customers){ [customer] }

      mrr.subscriptions.should eq([subscription])
    end
  end

  describe "#mrr" do
    it "should correctly tally the mrr from all its subscriptions" do
      mrr           = build_mrr
      plan          = double("plan", amount: 100)
      subscription  = double("subscription", plan: plan)

      allow(mrr).to receive(:subscriptions) { Array.new(3, subscription) }

      mrr.mrr.should eq(300)
    end
  end

  def build_mrr(api_key = 'some_api_key')
    Stripe::Mrr.new(api_key: api_key)
  end

  def customer_list(has_more = false, customers = [])
    OpenStruct.new(has_more: has_more, data: customers)
  end

  def new_customer
    Stripe::Customer.create
  end

  def new_subscription
    Stripe::Subscription.create
  end

end

This is awesome but not sure the math is right. I ran it on my account and received an incorrect value.