AssertIdentity has just been released. It’s a small helper library for ExUnit that provides assertions useful for comparing data structures by identity.

AssertIdentity really shines in applications that leverage Ecto. Consider the following:

def MyApp.PostTest do
  use MyApp.DataCase

  test "list_posts/0 returns all posts" do
    post = insert(:post)
    assert Posts.list_posts() == [post]
    assert_ids_match(Posts.list_posts(), [post])
  end
end

The first assertion fails when using factories that, for example, insert associations. (That’s because on the left-hand side you’ll have Ecto.Association.NotLoaded for the association values but on the right-hand side you’ll have a real association struct.)

The assert_ids_match/2 assertion passes because it instead plucks the :id key from each struct and compares those values (instead of an identity comparison on the whole struct).

Check out the docs for more examples and options on how to customize the identity assertion to your needs!

Links:

• Announcement blog post
• Hex
• Docs
• GitHub
• Development by NewAperio, expert Elixir/Phoenix consultants