def create_training(%{date: date, exercises: exercises}, user) do
    training_changeset = user
    |> Ecto.build_assoc(:trainings)
    |> Training.changeset(%{date: date})

    IO.inspect(training_changeset)

    Multi.new()
    |> Multi.insert(:training, training_changeset)
    |> Multi.insert(:exercise, fn %{training: training} ->
        Ecto.build_assoc(training, :exercises)
      end)
    |> Repo.transaction()
  end