Formalizes fetching the ID from data.

For maps the keys id, "id" are considered id fields and guid, "guid" are considered guid fields.

When requesting a guid, if one cannot be found by default it will fall back to the id.

Using with your own structs

By default your own structs will behave the same way as a map. However if you need to redefine which field should be considered the id/guid fields you'll need to derive the protocol.

  defmodule MyStruct do
    @derive {C3P0.ID, id_field: :token, guid_field: :arn}
    defstruct [:token, :arn, :name]
  end