From https://once.com/writebook

test/controllers/first_runs_controller_test.rb

@@ -0,0 +1,48 @@
+require "test_helper"
+
+class FirstRunsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    Book.destroy_all
+    User.destroy_all
+  end
+
+  test "new" do
+    get first_run_url
+    assert_response :success
+  end
+
+  test "new is not permitted when users exist" do
+    create_user
+
+    get first_run_url
+    assert_redirected_to root_url
+  end
+
+  test "create" do
+    assert_difference -> { User.count }, 1 do
+      post first_run_url, params: { user: { name: "New Person", email_address: "new@37signals.com", password: "secret123456" } }
+    end
+
+    assert_redirected_to root_url
+
+    assert parsed_cookies.signed[:session_token]
+  end
+
+  test "create is not permitted when users exist" do
+    create_user
+
+    assert_no_difference -> { User.count } do
+      post first_run_url, params: { user: { name: "New Person", email_address: "new@37signals.com", password: "secret123456" } }
+    end
+
+    assert_redirected_to root_url
+  end
+
+  private
+    def create_user
+      User.create! \
+        name: "Existing Person",
+        email_address: "user@example.com",
+        password: "secret123456"
+    end
+end