Seems like Elixir is matching a Ecto.Changeset.cast/3 into a Ecto.Changeset.cast/4 adding an empty array as the 4th parameter. This is happening in every test with ecto, is working fine in the API, I have no idea what is causing this.
I think the problem is with the first argument, actually. It should probably be a product struct instead of plain map in your case.
What does get_product_by_external_code/1 return? Since ProductRepo.update_product(product, valid_product_to_update) since to be getting two maps according to the stacktrace?
