create_invoice_spec.rb

describe CreateInvoice do
  
  subject { CreateInvoice.new(user, notification_center, form) }
    
  let(:user) { instance_double("User", subscriber?: true) }
  let(:notification_center) { instance_double("NotificationCenter", invoice_created: nil)
  let(:form) { instance_double("InvoiceForm", company_name: "ACME Corp.", billing_date: Time.now)

  describe "passing valid parameters" do

    it "is successful" do
      expect(subject.call).to be_success
    end
    
    it "creates invoice" do
      invoice = subject.call.invoice
      expect(invoice).to be_persisted
      expect(invoice.company_name).to eq("ACME Corp.")
    end
    
    it "notifies via notification center" do
      expect(notification_center).to receive(:invoice_created)
      subject.call
    end
  end
  
  describe "user is not a subscriber" do
    let(:user) { instance_double("User", subscriber?: false) }
    
    it "is unsuccessful" do
      expect(subject.call).not_to be_success
      expect(subject.error_message).to match("not a subscriber")
    end
  end
  
  # ... more failure scenarios
end