example_code.ex

defmodule MyAuthenticator do
  def auth(repo, model, id, password) do
    user =
      model
      |> where(id: ^id)
      |> preload([:profile])
      |> repo.one!()
  end
end

example_code_test.exs

defmodule MyAuthenticatorTest do
  use ExUnit.Case
  
  defmodule RaiseRepo do
    def one!(_),
      do: raise Ecto.NoResultsError
  end

  test "my code" do
    assert_raise Ecto.NoResultsError, fn ->
      MyAuthenticator.auth(RaiseRepo, User, 1, "password")
    end
  end
end