Waffle.Ecto connects your Waffle definition with Ecto via custom Ecto types.

First, connect Waffle.Ecto to your uploader definition

defmodule MyApp.AvatarUploader do
  use Waffle.Ecto.Definition

  def storage_dir(_, {_, scope}), do: "uploads/avatar/#{scope.id}"
end

Second, use the definition's Ecto-compatible type in your schema

defmodule MyApp.User do
  schema "users" do
    field :avatar, MyApp.AvatarUploader.Type
  end

  def changeset(user, attrs) do
    user
    |> cast_attachments(attrs, [:avatar])
  end
end

Please read Waffle.Ecto.Schema.cast_attachments/4 for details on how attachment casting works.

Also, Waffle.Ecto stores the updated_at timestamp in the file schema and automatically adds it to the URL for cache busting.

How to use schema id in storage directory

If you have auto-generated record IDs, you need to insert a record before processing the file upload.

Ecto.Multi.new()
|> Ecto.Multi.insert(:user, User.changeset(user, attrs))
|> Ecto.Multi.update(:user_with_avatar, &User.avatar_changeset(&1.user, attrs))
|> Repo.transaction()