Enough RSpec How to force the user as the serializer’s scope: # spec/serializers/book_serializer_spec.rb RSpec.describe BookSerializer do subject(:serialized_book) { serialize(book, scope: user) } let(:user) { create :user } let(:book) { ... } # test stuff end Based on how you define the it part, different behaviors are to be expected: # spec/serializers/things_controller_spec.rb RSpec.describe ThingsController do subject { post :create, params: params } # Implicitly calls subject it { is_expected.to redirect_to(thing_path(Thing.last)) } # Explicitly call subject it "redirects to the new thing's show" do expect(subject).to redirect_to(thing_path(Thing.last)) end # Does not call subject # For this test to work, a before { subject } is needed it 'has correct flash message' do expect(flash[:notice]).to eq "The thing has been created." end end