competition.ex

defmodule Api.Competition do
  @moduledoc """
    TODO: Write.
  """

  use Api.Web, :model

  alias Ecto.{Changeset}

  @valid_attrs ~w(
    voting_started_at
    voting_ended_at
  )a

  schema "competition" do
    field :voting_started_at, :utc_datetime
    field :voting_ended_at, :utc_datetime
  end

  defp _cant_change(%Changeset{changes: changes, data: data} = changeset, field) do
    with {:ok, old} <- Map.get(data, field),
         {:ok, new} <- Map.get(changes, field),
         old != nil && old != new
    do
      Changeset.add_error(changeset, field, "can't be changed")
    else
      _ -> changeset
    end
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, @valid_attrs)
    |> _cant_change(:voting_started_at)
    |> _cant_change(:voting_ended_at)
  end
end

competition_actions.ex

defmodule Api.CompetitionActions do
  use Api.Web, :action

  alias Api.{Competition}

  defp _change(params) do
    (Repo.one(from(c in Competition)) || %Competition{})
    |> Competition.changeset(params)
    |> Repo.insert_or_update
  end

  def open_voting do
    _change(%{
      voting_started_at: DateTime.utc_now()
    })
  end

  def close_voting do
    _change(%{
      voting_ended_at: DateTime.utc_now()
    })
  end
end

competition_actions_test.exs

defmodule Api.CompetitionActionsTest do
  use Api.ModelCase

  alias Api.{CompetitionActions}

  test "voting_ended_at set" do
    {:ok, _} = CompetitionActions.open_voting()
  end

  test "voting_ended_at set twice" do
    {:ok, _} = CompetitionActions.open_voting()
    {:error, _} = CompetitionActions.open_voting()
  end
end