Ecto: Microseconds not being discarded

[LostKobrakai]

Environment

• Elixir version (elixir -v): 1.7
• Database and version (PostgreSQL 9.4, MongoDB 3.2, etc.): PostgreSQL 10
• Ecto version (mix deps): 3.0-rc.1
• Database adapter and version (mix deps): Postgrex 2.0-rc
• Operating system: Mac OS X

Current behavior

Trying out ecto 3 and running my test suite I get hundreds of those errors:

** (Ecto.ChangeError) value `#DateTime<2018-10-21 23:32:31.464747Z>` for `Connect.Authentication.User.reset_sent_at`in `update` does not match type :utc_datetime

     Microseconds must be empty. Use `DateTime.truncate(utc_datetime, :second)` (available in Elixir v1.6+) to remove microseconds.

Expected behavior

Microseconds just being discarded without manual intervention.

[LostKobrakai]

As mentioned here this seems to be expected behavior. If so I'd suggest making the changelog more clear about it. Currently it says:

To uniformly support microseconds across all databases, the types :time, :naive_datetime, :utc_datetime will now discard any microseconds information.

This seems to be true for casting datetimes, but not when directly changing datetimes in the system without going through a casting process.

[AndrewDryga]

@LostKobrakai they are discarded if you use changeset (in cast/2), once you use something like Ecto.Changeset.change/2 or put_change/3 then Ecto assumes you work with an internal data structure and then it's up to you to discard microseconds.

[LostKobrakai]

I can see the reasoning behind the decision to have it that way, but as I said above it might be good to make people aware of that. At least in my application there where quite a bunch of the following functions, which were raising errors for a reason I though I wouldn't have to deal with.

def mark_published(struct), do: change(struct, %{published: DateTime.utc_now()})

[AndrewDryga]

I think if you submit a PR to the changelog Core Team would be happy to merge it :).

[LostKobrakai]

I've added a PR #2765 for changing the changelog here, but there are a few other places as well like the plataformatec blogposts, which would be nice if updated.

[josevalim]

As the error message says, this is by design. You either have to truncate

it or move to usec.

José Valim
www.plataformatec.com.br
Skype: jv.ptec
Founder and Director of R&D