Invoice changeset with constraint

invoice.ex

defmodule Registrar.Billing.Invoice do
  import Ecto.Changeset

  def changeset(%Invoice{} = invoice, attrs \\ %{}) do
    invoice
    |> cast(attrs, [
      :amount_due,
      :due_date,
      :scheduled_send_date,
      :email
      ])
    |> validate_required([
      :amount_due,
      :due_date,
      :scheduled_send_date,
      :email
    ])
    |> validate_due_date_not_past()
    |> unique_constraint(:email)
  end
end