Hi,
I am trying to set the value of a ‘time’ column to NULL using Ecto from a Phoenix app that interacts with a mssql database.
The problem comes when trying to update the value of a ‘time’ field to null (Inserts a struct is ok, even with null value).
I think it is not related to the original project because with a simple ‘ecto’ project the problem is the same: in the test project changing the database adapter to Postgres the problem disappears.
The test project is simple:
migration:

defmodule Hello.Repo.Migrations.CreateInfo do
  use Ecto.Migration

  def change do
    create table(:infos) do
      add :name, :string
      add :alias, :string
      add :simple_time, :time
      timestamps()
    end
    create(unique_index(:infos, :alias))
  end
end

schema:

defmodule Hello.Info do
  use Ecto.Schema
  import Ecto.Changeset

  schema "infos" do
    field(:alias, :string)
    field(:name, :string)
    field(:simple_time, :time)
    timestamps()
  end

  def changeset(info, params \\ %{}) do
    info
    |> cast(params, [:name, :alias, :simple_time])
    |> IO.inspect()
    |> validate_required([:name, :alias])
    |> unique_constraint(:alias)
  end
end

Insertion is ok even with null value.
Update causes the error with this message from test suite:

defmodule Hello.InfoTest do
  use Hello.RepoCase
...
   test "update time field to nil" do
    info = %Info{}
    changeset = Info.changeset(info, @valid_attrs)
    assert {:ok, info} = Repo.insert(changeset)

    info = Repo.get!(Info, info.id)

    {:ok, update_info} =
      info
      |> Info.changeset(%{simple_time: nil})
      |> Repo.update()

    assert update_info.simple_time == nil
  end
...

> mix test 
...
10:05:02.965 [debug] QUERY ERROR db=0.0ms queue=2.1ms
UPDATE [infos] SET [simple_time] = @1, [updated_at] = @2 WHERE [id] = @3 [nil, ~N[2021-05-27 08:05:02], 1]


  1) test update time field to nil (Hello.InfoTest)
     test/hello/info_test.exs:26
     ** (Tds.Error) Line 1 (Error 257): Implicit conversion from data type varbinary to time is not allowed. Use the CONVERT function to run this query.
     code: |> Repo.update()
     stacktrace:
       (ecto_sql 3.6.1) lib/ecto/adapters/sql.ex:749: Ecto.Adapters.SQL.raise_sql_call_error/1
       (ecto 3.6.1) lib/ecto/repo/schema.ex:717: Ecto.Repo.Schema.apply/4
       (ecto 3.6.1) lib/ecto/repo/schema.ex:426: anonymous fn/15 in Ecto.Repo.Schema.do_update/4
       test/hello/info_test.exs:36: (test)
...

any help is appreciated
Thanks in advance!