Inline Projections

An "inline" projection just means that Marten will process the projection against new events being appended to the event store at the time that IDocumentSession.SaveChanges() is called to commit a unit of work. Here's a small example projection:

public class MonsterDefeatedTransform: EventProjection
    public MonsterDefeated Create(IEvent<MonsterSlayed> input)
        return new MonsterDefeated
            Id = input.Id,
            Monster = input.Data.Name

public class MonsterDefeated
    public Guid Id { get; set; }
    public string Monster { get; set; }

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Note that the inline projection is able to use the event metadata at the time the inline projection is executed. That was previously a limitation of Marten that was fixed in Marten V4.

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

    opts.Projections.Add(new MonsterDefeatedTransform(),

using var session = store.LightweightSession();

var streamId = session.Events
    .StartStream<QuestParty>(started, joined, slayed1, slayed2, joined2).Id;

// The projection is going to be applied right here during
// the call to SaveChangesAsync() and the resulting document update
// of the new MonsterDefeated document will happen in the same database
// transaction
await theSession.SaveChangesAsync();

snippet source | anchor