Yes there is! I was looking into this the other day because I want to generate repeatable UUID’s for doc generation.

It works by setting the @primary_key module attribute :autogenerate option to a MFA. Here’s a short example:

defmodule MyApp.ExampleSchema do
  @primary_key {:id, Ecto.UUID, autogenerate: {MyApp.OIDGenerator, :generate_oid, []}}
  schema "example_schema" do
    # ... schema fields
  end
end

defmodule MyApp.OIDGenerator do
  def generate_oid do
    Ecto.UUID.generate()
    |> String.upcase()
    |> String.replace("-", "")
  end
end

And you could even put your @primary_key definition in a module and then use MyApp.Data to bring it in.