Setting up Arc in a phoenix app that was ported from a rails app using paperclip.

paperclip.ex

# Ecto - Model
defmodule YourPhoenixApp.PaperclipAvatar do
  use YourPhoenixApp.Web, :model
  alias YourPhoenixApp.{Repo, PaperclipAvatar}
  #
  # There are places in my existing app where only the avatar image is required.  
  # So I created a module that sole purpose was to read that data.
  #
  schema "users" do 
    field :avatar_file_name,      :string
    field :avatar_content_type,   :string
    field :avatar_file_size,      :integer
    field :avatar_updated_at,     Ecto.DateTime 
  end

  @_epoch :calendar.datetime_to_gregorian_seconds({{1970, 1, 1}, {0, 0, 0}})

  def paperclip_hash do 
    System.get_env("PAPERCLIP_HASH_SECRET")
  end
  
  def to_timestamp(datetime) do  
    datetime
    |> Ecto.DateTime.to_erl
    |> :calendar.datetime_to_gregorian_seconds 
    |> -@_epoch
  end

end

# Arc Uploader
defmodule YourPhoenixApp.Avatar do
  use Arc.Definition
  use Arc.Ecto.Definition
  alias YourPhoenixApp.{PaperclipAvatar, Repo}

  # Include ecto support (requires package arc_ecto installed):
  # use Arc.Ecto.Definition
  # Override the persisted filenames:
  @acl :public_read_write
  @versions [:original, :thumb]
  # To add a thumbnail version:
  # Whitelist file extensions:
  def validate({file, _}) do
    ~w(.jpg .jpeg .gif .png) |> Enum.member?(Path.extname(file.file_name))
  end

  # Define  a thumbnail transformation:
  # def transform(:thumb, _) do
  #   {:convert, "-strip -thumbnail 250x250^ -gravity center -extent 250x250 -format png"}
  # end

  # Override the persisted filenames:
  # def filename(version, _) do
  #   version
  # end
  def __storage, do: Arc.Storage.S3

  def storage_dir(style, base) do
    {_, legacy} = base
    case legacy do
      %YourPhoenixApp.PaperclipAvatar{} -> paperclip_url(style, legacy)
      _ -> _avatar_url_hash(style, legacy)
    end
  end

  defp _avatar_url_hash(style, legacy) do 
    ecto_date = case Mix.env do
      :dev  -> legacy.created_at
      :prod -> legacy.created_at
      :test  -> legacy.inserted_at
    end
    timestamp = ecto_date |> PaperclipAvatar.to_timestamp
    hash = "users/avatars/#{legacy.id}/#{style}/#{timestamp}"
      |> _encode |> String.downcase
    "users/#{hash}/#{legacy.id}/#{style}"
  end

  def paperclip_url(style, legacy) do
    coded = _storage_dir(style, legacy)
            |> _encode 
            |> String.downcase
    "users/#{coded}/#{legacy.id}/#{style}/"
  end

  defp _storage_dir(hash, legacy) do
    timestamp = paperclip_timestamp(legacy)
    "users/avatars/#{legacy.id}/#{hash}/#{timestamp}"
  end

  def paperclip_timestamp(%YourPhoenixApp.PaperclipAvatar{}=legacy) do 
    legacy.avatar_updated_at
    |> YourPhoenixApp.PaperclipAvatar.to_timestamp
  end

  def paperclip_timestamp(not_legacy), do: :os.system_time(:seconds)

  def _encode(url) do
    :crypto.hmac(:sha, System.get_env("PAPERCLIP_HASH_SECRET"), url) 
    |> Base.encode16
  end

end