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Ensuring Elixir Application Performance with Testing and ExUnit

Testing the Error Case for Create

Explore how to test failure scenarios for Phoenix create endpoints with ExUnit by asserting HTML error messages and verifying no new users or emails are created. Understand using Floki for precise HTML assertions and Mox for controlling side effects to keep your tests reliable and maintainable.

We'll cover the following...

An error test for create

We can complete our coverage of this endpoint and controller action by testing a failure case. Inside the same describe block POST /users, and under our previous test, add this:

Elixir
#file path -> test/not_skull_web/controllers/user_controller_test.exs
#add this code at the indicated place mentioned in comments of test/not_skull_web/
#controllers/user_controller_test.exs in the playground widget
test "error: does not insert, redirects to 'new' page w/invalid attributes",
         %{
           conn: conn
         } do
      flunk_if_email_is_sent()  
      expected_user_count = Repo.all(User) |> Enum.count() 
      conn = post(conn, Routes.user_path(conn, :create), user: %{})
      assert html = html_response(conn, 200)
      parsed_html = Floki.parse_document!(html)
      for field <- ["name", "email", "password"] do 
        # using <> so that the first #(indicating an id) is easy to read
        field_as_id = "#" <> "#{field}-error"
        span = Floki.find(parsed_html, field_as_id)
        assert Floki.text(span) == "can't be blank" 
      end
      assert Repo.all(User) |> Enum.count() == expected_user_count, 
             "There should have been no records inserted during this test."
end

Let’s see the details of the test:

  • The test leverages a new custom helper function called flunk_if_email_is_sent/0, on line 8, that’s defined in NotSkull.ConnCase, right next to our other helper function.

Note: Because our test is a failure case, it needs to make sure that no new users were created as a side effect.

  • On line 10, the test is getting the pre-exercise count of users in the database. This ...