Set up Ecto to run test in Sanbox mode

my_app_test.exs

defmodule MyApp.Test do
  use ExUnit.Case, async: true
  use Plug.Test

  setup do
    # Checkout repo manually.
    # TODO: figure out how to do it automatically.
    :ok = Ecto.Adapters.SQL.Sandbox.checkout(MyApp.Repo)
  end
  
  # Test your business here.
end

test.exs

use Mix.Config

config(
  :my_app,
  MyApp.Repo,
  adapter: Ecto.Adapters.Postgres,
  pool: Ecto.Adapters.SQL.Sandbox, # This is needed to run in Sandbox mode.
  database: "database-test",
  hostname: "localhost"
)

test_helper.exs

{:ok, _} = Application.ensure_all_started(:ex_machina)

Ecto.Adapters.SQL.Sandbox.mode(MyApp.Repo, :manual) # This is needed to run in Sanbox mode.

ExUnit.start()

You shouldn't do that. Everything is already executed in a transaction and cleaned by itself. I suppose you are running a seed task before to run your tests, in that case ok. Still you could just check with some conditions if the database is already seeded in your seed file ;)