Tip: Using Ecto transactions in tests

Whenever tests have to interact with an Ecto.Repo, sometimes it’s necessary to test a few different branches or paths that data can take. This can be tedious with a database and may result in “resetting” records so they may be re-tested. Here’s a crude example of “resetting” some data to continue with a test:

test "vaccinate_dog/1 vaccinates dog and creates vax record with the corresponding name" do
  dog = dog_fixture()
  
  # Perform assertions about vaccinations
  assert :ok = vaccinate_dog(dog, "bordetella")
  assert %{id: dog_id, vaccinated: true} = dog = Repo.one!(Dog)
  assert %{dog_id: ^dog_id, type: "bordetella"} = Repo.one!(Vax)

  # Reset state for the dog
  Repo.delete_all(Vax)
  dog |> Ecto.Changeset.change(%{vaccinated: false}) |> Repo.update!()

  # Performmore  assertions about vaccinations
  assert :ok = vaccinate_dog(dog, "rabies")
  assert %{id: dog_id, vaccinated: true} = dog = Repo.one!(Dog)
  assert %{dog_id: ^dog_id, type: "rabies"} = Repo.one!(Vax)
end

By using Repo.transaction/2, it’s easy to simply roll back a transaction rather than “resetting” anything (assuming that side-effects are not a concern for the purposes of the test.

test "vaccinate_dog/1 vaccinates dog and creates vax record with the corresponding name" do
  dog = dog_fixture()

  for vaccine <- ["rabies", "bordetella"] do
    in_transaction(fn ->
      assert :ok = vaccinate_dog(dog, vaccine)
      assert %{id: dog_id, vaccinated: true} = dog = Repo.one!(Dog)
      assert %{dog_id: ^dog_id, type: vaccine} = Repo.one!(Vax)
    end)
  end
  
  defp in_transaction(fun) when is_function(fun, 0) do
    Repo.transaction(fn ->
      fun.()
      Repo.rollback(:ok)
    end)
  end
end

I know that it’s generally best to decouple the DB and the business logic as much as possible, but I have found this to be a useful way to succinctly cover more ground within Repo-related tests.