Here are the requirements:

• If initial_invoice? is false, existing_invoice always exists.
• If initial_invoice? is true and existing_invoice exists, I'd love to use the existing_invoice.

I'd love to refactor this code.

 existing_invoice = find_existing_invoice(invoice_obj[:id])
   invoice = if !initial_invoice?(invoice_obj) || existing_invoice.present?
                existing_invoice
              else
                ac = Account.find_by(stripe_customer_id: invoice_obj[:customer])
                create_initial_invoice(invoice_obj, ac)
              end

  private

  def find_existing_invoice(id)
    Invoice.find_by(stripe_invoice_id: id)
  end

  def initial_invoice?(invoice_obj)
    sub_id = invoice_obj[:subscription]
    return false if sub_id.nil?

    sub = Stripe::Subscription.retrieve(sub_id)
    return false unless invoice_obj[:period_start] == sub[:created]
    true
  end