Why does dumping DateTime to :utc_datetime_usec with a precision less than 6 fail?

Question

I am wondering why I can't dump a DateTime with a precision less than 6? Why can't we just fill up with zeros?

Example

iex> {:ok, datetime, _} = DateTime.from_iso8601("1970-01-01T00:00:00Z")
{:ok, #DateTime<1970-01-01 00:00:00Z>, 0}

Expected

iex> Ecto.Type.dump(:utc_datetime_usec, datetime)
{:ok, #DateTime<1970-01-01 00:00:00.000000Z>}

Actual

iex> Ecto.Type.dump(:utc_datetime_usec, datetime)
** (ArgumentError) :utc_datetime_usec expects microsecond precision, got: #DateTime<1970-01-01 00:00:00Z>
    (ecto) lib/ecto/type.ex:1217: Ecto.Type.check_usec!/2
    (ecto) lib/ecto/type.ex:419: Ecto.Type.dump_utc_datetime_usec/1

Answer 1

As the type name says, utc_datetime_usec does expect usec precision. So giving it anything with less precision will make it fail. You can use utc_datetime if you don't want to enforce microseconds.