Example of lazy execution in rspec - let vs. let! vs. object mention

delete_test_3.rb

require 'rails_helper'

RSpec.describe V1::AppsController, type: :controller do
  let(:user) { create(:user) }

  describe '[mention] DELETE destroy when you are not logged in' do
    let(:app) { create(:app, account: user.account) }
    let(:make_request) { delete :destroy, id: app, format: :json }

    it 'does not delete the app (.count)' do
      app
      expect { make_request }.not_to change(App, :count)
    end
  end

  describe '[let] DELETE destroy when you are not logged in' do
    let(:app) { create(:app, account: user.account) }
    let(:make_request) { delete :destroy, id: app, format: :json }

    it 'does not delete the app (.count)' do
      expect { make_request }.not_to change(App, :count)
    end
  end

  describe '[let!] DELETE destroy when you are not logged in' do
    let!(:app) { create(:app, account: user.account) }
    let(:make_request) { delete :destroy, id: app, format: :json }

    it 'does not delete the app (.count)' do
      expect { make_request }.not_to change(App, :count)
    end
  end
end

rspec output

V1::AppsController
  [mention] DELETE destroy when you are not logged in
    does not delete the app (.count)
  [let] DELETE destroy when you are not logged in
    **** does not delete the app (.count) (FAILED - 1) ****
  [let!] DELETE destroy when you are not logged in
    does not delete the app (.count)

Failures:

  1) V1::AppsController [let] DELETE destroy when you are not logged in does not delete the app (.count)
     Failure/Error: expect { make_request }.not_to change(App, :count)
       expected #count not to have changed, but did change from 0 to 1
     # ./spec/controllers/v1/delete_test_3.rb:21:in `block (3 levels) in <top (required)>'

Finished in 0.85543 seconds (files took 3.77 seconds to load)
3 examples, 1 failure

Failed examples: