data_case.ex

defmodule MyApp.DataCase do
  use ExUnit.CaseTemplate

  using do
    quote do
      import Commanded.Assertions.EventAssertions
    end
  end

  setup do
    {:ok, _} = Application.ensure_all_started(:my_app)

    on_exit(fn ->
      :ok = Application.stop(:my_app)
      :ok = Application.stop(:commanded)
      :ok = Application.stop(:eventstore)

      MyApp.Storage.reset!()
    end)

    :ok
  end
end

storage.ex

defmodule MyApp.Storage do
  @doc """
  Clear the event store and read store databases
  """
  def reset! do
    reset_eventstore()
    reset_readstore()
  end

  defp reset_eventstore do
    config = EventStore.Config.parsed() |> EventStore.Config.default_postgrex_opts()

    {:ok, conn} = Postgrex.start_link(config)

    EventStore.Storage.Initializer.reset!(conn)
  end

  defp reset_readstore do
    config = Application.get_env(:my_app, MyApp.Repo)

    {:ok, conn} = Postgrex.start_link(config)

    Postgrex.query!(conn, truncate_readstore_tables(), [])
  end

  defp truncate_readstore_tables do
    """
    TRUNCATE TABLE
      foo,
      bar,
      baz
    RESTART IDENTITY
    CASCADE;
    """
  end
end