explicit.rb

test "Can't register with a username that has been taken" do
  create(:user, username: "already-taken")
  
  assert_raises(ActiveRecord::RecordNotSaved) do
    post "/register", username: "already-taken"
  end
end

implicit.rb

test "can access dotted-resources with dashes" do
  create(:resource, path: "one.two")
  
  get "/resources/one-two"
  # raised an ActiveRecord::RecordNotFound
end