Projecting by Event Type

While projections can target a specific stream or streams, it is also possible to project by event type. The following sample demonstrates this with a CandleProjection that extends MultiStreamProjection<Candle, Guid> to build Candle aggregates from events of type Tick, grouping every Tick by its CandleId regardless of which stream the event was captured in.

Introduce a type to hold candle data:

public class Candle
{
    public Guid Id { get; set; }
    public decimal Open { get; set; }
    public decimal High { get; set; }
    public decimal Low { get; set; }
    public decimal Close { get; set; }
}

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This data will then be populated and updated from observing ticks:

// Each Tick carries the identity of the candle it contributes to. The
// ticks can be captured in any number of separate event streams.
public record Tick(Guid CandleId, decimal Price);

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We then introduce a projection that subscribes to the Tick event:

public partial class CandleProjection: MultiStreamProjection<Candle, Guid>
{
    public CandleProjection()
    {
        // Group every Tick event by its CandleId so that all ticks for the
        // same candle are aggregated together, regardless of which stream
        // each Tick was captured in. The projection only reacts to events
        // of type Tick.
        Identity<Tick>(x => x.CandleId);
    }

    public void Apply(Candle candle, Tick tick)
    {
        if (candle.Open == 0)
        {
            candle.Open = tick.Price;
        }

        candle.High = candle.High == 0 ? tick.Price : Math.Max(candle.High, tick.Price);
        candle.Low = candle.Low == 0 ? tick.Price : Math.Min(candle.Low, tick.Price);
        candle.Close = tick.Price;
    }
}

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Lastly, we configure the event store to use the newly introduced projection:

var store = DocumentStore.For(opts =>
{
    opts.Connection("some connection string");

    // Register the projection by event type
    opts.Projections.Add<CandleProjection>(ProjectionLifecycle.Inline);
});

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