Testing model scope

How can I test a scope of my Subscription model.

class Subscription < ApplicationRecord

  has_many invoices, as invoiceable

  scope :total, -> { joins(:invoices).where(invoices: Invoice.month(2.month.ago)).where.not(invoices: Invoice.month(1.month.ago)) }

end

These are the fields of invoice

    t.string "invoiceable_type"
    t.bigint "invoiceable_id"
    t.string "status", default: "draft"
    t.string "number"
    t.string "period"

The scope takes the subcriptions that has invoices two month ago, but has not invoice last month.