Problem trying to call Ecto.DateTime.to_iso8601

Hi everyone!

I’m trying to follow the tutorial from thoughtbot here to create my first phoenix json api endpoint. The code for the test is following:

# Create this test in test/controllers/todo_controller_test.exs
defmodule Todos.TodoControllerTest do
  use Todos.ConnCase

  test "#index renders a list of todos" do
    conn = build_conn()
    todo = insert(:todo)

    conn = get conn, todo_path(conn, :index)

    assert json_response(conn, 200) == %{
      "todos" => [%{
        "title" => todo.title,
        "description" => todo.description,
        "inserted_at" => Ecto.DateTime.to_iso8601(todo.inserted_at),
        "updated_at" => Ecto.DateTime.to_iso8601(todo.updated_at)

When I run the test I get the following error:

 ** (FunctionClauseError) no function clause matching in Ecto.DateTime.to_iso8601/1
       (ecto) lib/ecto/date_time.ex:584: Ecto.DateTime.to_iso8601(~N[2017-03-23 19:55:47.300335])
       test/controllers/todo_controller_test.exs:14: (test)

I’ve completed the tutorial up to the point where this should be passing. If I change the test so that I’m not attempting to format the date I get the following:

     Assertion with == failed
     code: json_response(conn, 200) == %{"todos" => [%{"title" => todo.title(), "description" => todo.description(), "inserted_at" => "Frank", "updated_at" => "Frank"}]}
     lhs:  %{"todos" => [%{"description" => "List of steps I need to complete", "title" => "Something I need to do", "inserted_at" => "2017-03-23T19:56:39.484442", "updated_at" => "2017-03-23T19:56:39.497129"}]}
     rhs:  %{"todos" => [%{"description" => "List of steps I need to complete", "title" => "Something I need to do", "inserted_at" => "Frank", "updated_at" => "Frank"}]}
       test/controllers/todo_controller_test.exs:10: (test)

Which seems to make sense. The problem is with Ecto.DateTime.to_iso and it’s no matching clause. The ecto code is this:

  def to_iso8601(%Ecto.DateTime{year: year, month: month, day: day,
                                hour: hour, min: min, sec: sec, usec: usec}) do
    str = zero_pad(year, 4) <> "-" <> zero_pad(month, 2) <> "-" <> zero_pad(day, 2) <> "T" <>
          zero_pad(hour, 2) <> ":" <> zero_pad(min, 2) <> ":" <> zero_pad(sec, 2)

    if is_nil(usec) or usec == 0 do
      str <> "." <> zero_pad(usec, 6)

So possibly the ~N[2017-03-23 19:57:48.500354] is not matching with the

%Ecto.DateTime{year: year, month: month, day: day,
               hour: hour, min: min, sec: sec, usec: usec}

in the ecto code? It is probably something small that I’m unaware of but I’ve very new to elixir and phoenix and would appreciate any advice on how to resolve this. Thanks so much for your time!


Having looked at it only a little bit it seems you’re creating a NaiveDateTime and what this function is looking for is actually a DateTime.

Ecto.DateTime has a function called cast/1 that you can use to convert it to an actual DateTime. You can then pass that to the function you’re trying to use. Note, though, that what you have in your sigil is what you’re going to get back.

Also, I have to ask: Who is Frank and how did he end up in your {inserted,updated}_at fields?


Gonz Thanks! I’ll check out that cast/1 I was just trying to figure out what the ~N[2017-03-23 20:58:23.586989] was all about. Is NaiveDateTime what is automatically used in Phoenix or is it configurable?

Frank is no one I know, just something to trip the test up so I can see what it gives me.

Obviously I have a lot of ground to cover in my knowledge in Elixir and Phoenix. Thanks for the tips!

1 Like

If it is a NaiveDateTime, then why not use NaiveDateTime.to_iso8601/1?

iex> NaiveDateTime.to_iso8601(~N[2017-03-23 20:58:23.586989])
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That works perfectly. Awesome tip, removes an extra step converting and discarding the :ok in the tuple that comes back from cast/1.


In my rush I looked for this in the Ecto namespace instead of checking the Elixir namespace.


I believe it’s the default timestamp type used by Ecto. You should be able to set it in your schema by using the following in your schema:

@timestamps_opts [type: :utc_datetime]

You can find more info here.

The difference being that DateTime (which corresponds to :utc_datetime) provides timezone info, which makes it more usable for conversions, etc.

There have been some great talks and work on this by Lau Taarnskov ((“lau” on GitHub). I recommend them, as it’s a surprisingly interesting area with a lot of pitfalls.


Great info thanks so much. I can see now how I got myself into the confusion by using timestamps which defaults to NaiveDateTime