category_stream_linker.ex

defmodule EventStore.CategoryStreamLinker do
  @moduledoc """
    Links streams from aggregate instances to their respective category streams.

    example: events from stream_uuid of `contractors_contract-07c52787-da0c-444f-9783-5d380f7093f9` will be
    linked to stream_uuid of `contractors_contract`.
  """

  use Commanded.Event.Handler,
    application: My.App,
    name: __MODULE__

  @default_opts [enabled: true]

  def handle(_event, metadata) do
    :my_app
    |> Application.get_env(:category_projections, @default_opts)
    |> Keyword.merge(@default_opts, fn _key, left, _right -> left end)
    |> Enum.into(%{})
    |> link_to_category(metadata)

    :ok
  end

  defp link_to_category(%{enabled: true}, %{stream_id: stream_id, event_id: event_id}) do
    stream_id
    |> String.split("-")
    |> link_to_category(event_id)
  end

  defp link_to_category([category | _], event_id) do
    MyApp.EventStore.link_to_stream(category, :any_version, [event_id])
  end

  defp link_to_category(_, _), do: nil
end