I’ve seen a few examples on using Ecto.Schema for validations but I can’t seem to find a way to make defaults play nicely. For some background, I’m building a API that aggregate and cleans data before POSTing it to an external HTTP service. Since the external service has tons of optional params and others that I can provide defaults to I wanted to use a_emphasized text_ schema.

defmodule Lead do
  use Ecto.Schema

  import Ecto.Changeset

  @required_fields ~w(device type_id website)a

  @primary_key false
  embedded_schema do
    field :device, :string
    field :type_id, :integer, default: 0
    field :session_id, :string
    field :website, :string, default: "facebook"
    field :message, :string
  end

  def changeset(%__MODULE__{} = lead, params \\ %{}) do
    lead
    |> cast(params, __schema__(:fields))
    |> cast_session_id
    |> validate_required(@required_fields)
    |> validate_inclusion(:device, ["web", "mobile"])
    |> validate_inclusion(:type_id, 0..6)
  end

  defp cast_session_id(changeset) do
    case get_field(changeset, :session_id) do
      nil ->
        unix_time =
          DateTime.utc_now
          |> DateTime.to_unix(:milliseconds)

        put_change(changeset, :session_id, "no_session_id_#{unix_time}")

      _ -> changeset
    end
  end
end

This works fine for the most part but when trying to get the final map I run into some weirdness.

iex(1)> cs = %Lead{} |> Lead.changeset(%{device: "web"})
#Ecto.Changeset<action: nil,
 changes: %{device: "web", session_id: "no_session_id_1503097557106"},
 errors: [], data: #Lead<>, valid?: true>

iex(2)> cs.changes
%{device: "web", session_id: "no_session_id_1503097557106"}

iex(3)> Ecto.Changeset.apply_changes(cs)
%Lead{device: "web", message: nil, session_id: "no_session_id_1503097557106",
 type_id: 0, website: "facebook"}

If I try and use the changes, then field defaults don’t work.
If I try to apply_changes I get fields with nil.

I see two options, I could either move the default into a cast like cast_session_id or I could just filter out keys with nil values. The real schema has 15 optional fields with 5 or so fields that I could provide defaults.

Thoughs / Suggestions?