Datetime2 Type for Ecto

date_time_2.ex

defmodule Ecto.Adapter.MSSQL.DateTime2 do
  @moduledoc """
  Temporary solution to fix microsecond precision in MSSQL UTC datetime2
  values.  MSSQL defaults to 7-digit microseconds, whereas `Calendar` only
  supports 6.

  Note: Ecto 3.0 will remove the microseconds by default with `:utc_datetime`.

  See https://github.com/livehelpnow/tds_ecto/issues/65 for more info.
  """

  @behaviour Ecto.Type

  @max_usec_precision 6

  def type, do: :utc_datetime_usec

  def cast(%DateTime{time_zone: "Etc/UTC"} = datetime), do: {:ok, datetime}
  def cast(_value), do: :error

  @doc "Load from database, fixes microseconds by ensuring theyre 6-digits"
  @spec load(any()) :: {:ok, DateTime.t()}
  def load(%DateTime{} = datetime) do
    {:ok, fix_usec_precision(datetime)}
  end

  def load(%NaiveDateTime{} = naive_datetime) do
    naive_datetime |> fix_usec_precision() |> DateTime.from_naive("Etc/UTC")
  end

  def load({{year, month, day}, {hour, minute, second, microsecond}}) do
    %DateTime{
      year: year,
      month: month,
      day: day,
      hour: hour,
      minute: minute,
      second: second,
      microsecond: {microsecond, 6},
      utc_offset: 0,
      std_offset: 0,
      time_zone: "Etc/UTC",
      zone_abbr: "UTC"
    }
    |> fix_usec_precision()
    |> load()
  end

  def dump(%DateTime{time_zone: time_zone} = datetime) do
    if time_zone != "Etc/UTC" do
      message =
        ":utc_datetime_usec expects the time zone to be \"Etc/UTC\", got `#{inspect(datetime)}`"

      raise ArgumentError, message
    end

    fixed_datetime =
      datetime
      |> fix_usec_precision()

    # tds_ecto expects the following format
    {
      :ok,
      {
        {fixed_datetime.year, fixed_datetime.month, fixed_datetime.day},
        {
          fixed_datetime.hour,
          fixed_datetime.minute,
          fixed_datetime.second,
          elem(fixed_datetime.microsecond, 0)
        }
      }
    }
  end

  defp fix_usec_precision(%{microsecond: {microsecond, _}} = struct) do
    fixed_microsecond =
      microsecond
      |> to_string
      |> String.split_at(@max_usec_precision)
      |> elem(0)
      |> String.to_integer()

    %{struct | microsecond: {fixed_microsecond, @max_usec_precision}}
  end

  defp fix_usec_precision(datetime), do: datetime

  # `Ecto.Repo.Schema.autogenerate_changes/3` expects this to be defined, not sure why.
  def from_unix!(microseconds, :microseconds),
    do: DateTime.from_unix!(microseconds, :microseconds)
end

defmodule Ecto.Adapter.MSSQL.DateTime2Test do
  use ExUnit.Case

  alias Ecto.Adapter.MSSQL.DateTime2, as: Type

  setup do
    {
      :ok,
      mssql_datetime: %DateTime{
        year: 2018,
        month: 8,
        day: 17,
        hour: 21,
        minute: 34,
        second: 14,
        microsecond: {86_857_499_999_999_999, 6},
        time_zone: "Etc/UTC",
        zone_abbr: "UTC",
        utc_offset: 0,
        std_offset: 0
      },
      datetime: %DateTime{
        year: 2018,
        month: 8,
        day: 17,
        hour: 21,
        minute: 34,
        second: 14,
        microsecond: {868_574, 6},
        time_zone: "Etc/UTC",
        zone_abbr: "UTC",
        utc_offset: 0,
        std_offset: 0
      }
    }
  end

  test "loads datetime", context do
    assert {:ok, context.datetime} == Type.load(context.datetime)
  end

  test "dumps datetime while truncating microseconds", context do
    assert {:ok, {{2018, 8, 17}, {21, 34, 14, 868_574}}} = Type.dump(context.mssql_datetime)
  end

  test "casts datetime", context do
    assert {:ok, context.datetime} == Type.cast(context.datetime)
  end
end