Read-side projection that folds events into state on every call.

Unlike a persistent projection, no state is stored between calls — events are re-read and re-folded each time run/3 is invoked. This is simple and always consistent, at the cost of re-reading events on every request.

The query/1 callback may use Query.limit/2 and Query.reverse/1, making it suitable for "last N events" patterns (e.g. checking whether a book is currently borrowed by fetching only the most recent event).

use Counterpoint.OnDemandProjection injects @behaviour Counterpoint.OnDemandProjection.

Example

defmodule MyApp.Views.OrderSummary do
  use Counterpoint.OnDemandProjection
  alias Counterpoint.Query
  alias MyApp.Events.{OrderPlaced, OrderCancelled}

  defstruct [:order_id, :status]

  @impl Counterpoint.OnDemandProjection
  def query(order_id) do
    Query.new()
    |> Query.add_item(types: [OrderPlaced, OrderCancelled], tags: ["order_id:#{order_id}"])
  end

  @impl Counterpoint.OnDemandProjection
  def init, do: %__MODULE__{}

  @impl Counterpoint.OnDemandProjection
  def apply(state, %Counterpoint.Envelope{data: %OrderPlaced{order_id: id}}),
    do: %{state | order_id: id, status: :placed}

  def apply(state, %Counterpoint.Envelope{data: %OrderCancelled{}}),
    do: %{state | status: :cancelled}
end

Counterpoint.OnDemandProjection.run(MyApp.Views.OrderSummary, :my_store, "order-1")