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Aggregate Projections

Aggregate Projections in Marten combine some sort of grouping of events and process them to create a single aggregated document representing the state of those events. To jump into a simple example, here's a simple aggregated view called QuestParty that creates an aggregated view of MembersJoined, MembersDeparted, and QuestStarted events related to a group of heroes traveling on a quest in your favorite fantasy novel:

public class QuestParty
{
    public List<string> Members { get; set; } = new();
    public IList<string> Slayed { get; } = new List<string>();
    public string Key { get; set; }
    public string Name { get; set; }

    // In this particular case, this is also the stream id for the quest events
    public Guid Id { get; set; }

    // These methods take in events and update the QuestParty
    public void Apply(MembersJoined joined) => Members.Fill(joined.Members);
    public void Apply(MembersDeparted departed) => Members.RemoveAll(x => departed.Members.Contains(x));
    public void Apply(QuestStarted started) => Name = started.Name;

    public override string ToString()
    {
        return $"Quest party '{Name}' is {Members.Join(", ")}";
    }
}

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Once again, here's the class diagram of the key projection types inside of Marten, but please note the AggregationProjection<T>:

Projection Class Diagram

Marten supports a few different types of aggregated projections:

  • Aggregates by Stream -- creating a rolled up view of all or a segment of the events within an event stream. This is done through either a self-aggregate or by using AggregateStream<T> as a base class for your projection.
  • Aggregates across Streams -- creating a rolled up view of a user-defined grouping of events across streams. These projections are done by sub-classing the ViewProjection<TDoc, TId> class and is further described in View Projections.

Please note that all aggregated projections share the same set of method conventions described in this page.

Aggregate by Stream

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Projection types and the associated aggregate types need to be scoped as public because of Marten's internal code generation techniques. Some methods discovered by the method conventions can be internal or private, but the holding type must be public.

The easiest type of aggregate to create is a document that rolls up the state of a single event stream. You can do that by either creating a public aggregate document that directly mutates itself through method conventions or by sub-classing the AggregateProjection<T> class like this sample for a fictional Trip aggregate document:

public class TripProjection: SingleStreamAggregation<Trip>
{
    public TripProjection()
    {
        DeleteEvent<TripAborted>();

        DeleteEvent<Breakdown>(x => x.IsCritical);

        DeleteEvent<VacationOver>((trip, v) => trip.Traveled > 1000);

        // Now let's change the lifecycle to inline
        Lifecycle = ProjectionLifecycle.Inline;
    }

    // These methods can be either public, internal, or private but there's
    // a small performance gain to making them public
    public void Apply(Arrival e, Trip trip) => trip.State = e.State;
    public void Apply(Travel e, Trip trip) => trip.Traveled += e.TotalDistance();
    public void Apply(TripEnded e, Trip trip)
    {
        trip.Active = false;
        trip.EndedOn = e.Day;
    }

    public Trip Create(TripStarted started)
    {
        return new Trip {StartedOn = started.Day, Active = true};
    }
}

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And register that projection like this:

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

    // Register as inline
    opts.Projections.Add<TripProjection>(ProjectionLifecycle.Inline);

    // Or instead, register to run asynchronously
    opts.Projections.Add<TripProjection>(ProjectionLifecycle.Async);
});

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Any projection based on AggregateProjection<T> will allow you to define steps by event type to either create, delete, or mutate an aggregate document through a mix of inline Lambda expressions in the constructor function of the projection class or by using specially named methods on the projection class. It's completely up to your preference to decide which to use.

Alternatively, if your aggregate will never be deleted you can use a "self-aggregate" as explained in the last section of this page.

To create aggregate projections that include events in multiple streams, see View Projections.

Aggregate Creation

Aggregates can initially be created behind the scenes by Marten if there's a no-arg constructor function on the aggregate document type -- which doesn't have to be public by the way.

You can also use a constructor that takes an event type as shown in this sample of a Trip self-aggregate:

public class Trip
{
    // Probably safest to have an empty, default
    // constructor unless you can guarantee that
    // a certain event type will always be first in
    // the event stream
    public Trip()
    {
    }

    // Create a new aggregate based on the initial
    // event type
    internal Trip(TripStarted started)
    {
        StartedOn = started.Day;
        Active = true;
    }

    public Guid Id { get; set; }
    public int EndedOn { get; set; }

    public double Traveled { get; set; }

    public string State { get; set; }

    public bool Active { get; set; }

    public int StartedOn { get; set; }
    public Guid? RepairShopId { get; set; }

    // The Apply() methods would mutate the aggregate state
    internal void Apply(Arrival e) => State = e.State;
    internal void Apply(Travel e) => Traveled += e.TotalDistance();
    internal void Apply(TripEnded e)
    {
        Active = false;
        EndedOn = e.Day;
    }

    // We think self-aggregates are mostly useful for live aggregations,
    // but hey, if you want to use a self aggregate as an asynchronous projection,
    // you can also specify when the aggregate document should be deleted
    internal bool ShouldDelete(TripAborted e) => true;
    internal bool ShouldDelete(Breakdown e) => e.IsCritical;
    internal bool ShouldDelete(VacationOver e) => Traveled > 1000;
}

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Or finally, you can use a method named Create() on a projection type as shown in this sample:

public class TripProjection: SingleStreamAggregation<Trip>
{
    public TripProjection()
    {
        DeleteEvent<TripAborted>();

        DeleteEvent<Breakdown>(x => x.IsCritical);

        DeleteEvent<VacationOver>((trip, v) => trip.Traveled > 1000);

        // Now let's change the lifecycle to inline
        Lifecycle = ProjectionLifecycle.Inline;
    }

    // These methods can be either public, internal, or private but there's
    // a small performance gain to making them public
    public void Apply(Arrival e, Trip trip) => trip.State = e.State;
    public void Apply(Travel e, Trip trip) => trip.Traveled += e.TotalDistance();
    public void Apply(TripEnded e, Trip trip)
    {
        trip.Active = false;
        trip.EndedOn = e.Day;
    }

    public Trip Create(TripStarted started)
    {
        return new Trip {StartedOn = started.Day, Active = true};
    }
}

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The Create() method has to return either the aggregate document type or Task<T> where T is the aggregate document type. There must be an argument for the specific event type or Event<T> where T is the event type if you need access to event metadata. You can also take in an IQuerySession if you need to look up additional data as part of the transformation or IEvent in addition to the exact event type just to get at event metadata.

Applying Changes to the Aggregate Document

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Apply() methods or ProjectEvent<T>() method calls can also use interfaces or abstract types that are implemented by specific event types, and Marten will apply all those event types that can be cast to the interface or abstract type to that method when executing the projection.

To make changes to an existing aggregate, you can either use inline Lambda functions per event type with one of the overloads of ProjectEvent():

public class TripProjection: SingleStreamAggregation<Trip>
{
    public TripProjection()
    {
        ProjectEvent<Arrival>((trip, e) => trip.State = e.State);
        ProjectEvent<Travel>((trip, e) => trip.Traveled += e.TotalDistance());
        ProjectEvent<TripEnded>((trip, e) =>
        {
            trip.Active = false;
            trip.EndedOn = e.Day;
        });

        ProjectEventAsync<Breakdown>(async (session, trip, e) =>
        {
            var repairShop = await session.Query<RepairShop>()
                .Where(x => x.State == trip.State)
                .FirstOrDefaultAsync();

            trip.RepairShopId = repairShop?.Id;
        });
    }
}

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I'm not personally that wild about using lots of inline Lambdas like the example above, and to that end, Marten now supports the Apply() method convention. Here's the same TripProjection, but this time using methods to mutate the Trip document:

public class TripProjection: SingleStreamAggregation<Trip>
{
    public TripProjection()
    {
        DeleteEvent<TripAborted>();

        DeleteEvent<Breakdown>(x => x.IsCritical);

        DeleteEvent<VacationOver>((trip, v) => trip.Traveled > 1000);

        // Now let's change the lifecycle to inline
        Lifecycle = ProjectionLifecycle.Inline;
    }

    // These methods can be either public, internal, or private but there's
    // a small performance gain to making them public
    public void Apply(Arrival e, Trip trip) => trip.State = e.State;
    public void Apply(Travel e, Trip trip) => trip.Traveled += e.TotalDistance();
    public void Apply(TripEnded e, Trip trip)
    {
        trip.Active = false;
        trip.EndedOn = e.Day;
    }

    public Trip Create(TripStarted started)
    {
        return new Trip {StartedOn = started.Day, Active = true};
    }
}

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The Apply() methods can accept any combination of these arguments:

  1. The actual event type
  2. Event<T> where the T is the actual event type. Use this if you want access to the event metadata like versions or timestamps.
  3. IEvent access the event metadata. It's perfectly valid to accept both IEvent for the metadata and the specific event type just out of convenience.
  4. IQuerySession if you need to do additional data lookups
  5. The aggregate type

The valid return types are:

  1. void if you are mutating the aggregate document
  2. The aggregate type itself, and this allows you to use immutable aggregate types
  3. Task if you are mutating the aggregate document with the use of external data read through IQuerySession
  4. Task<T> where T is the aggregate type. This allows you to use immutable aggregate types while also using external data read through IQuerySession

Deleting the Aggregate Document

In asynchronous or inline projections, receiving a certain event may signal that the projected document is now obsolete and should be deleted from document storage. If a certain event type always signals a deletion to the aggregated view, you can use this mechanism inside of the constructor function of your aggregate projection type:

public class TripAggregation: SingleStreamAggregation<Trip>
{
    public TripAggregation()
    {
        // The current Trip aggregate would be deleted if
        // the projection encountered a TripAborted event
        DeleteEvent<TripAborted>();
    }
}

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If the deletion of the aggregate document needs to be done by testing some combination of the current aggregate state, the event, and maybe even other document state in your Marten database, you can use more overloads of DeleteEvent() as shown below:

public class TripAggregation: SingleStreamAggregation<Trip>
{
    public TripAggregation()
    {
        // The current Trip aggregate would be deleted if
        // the Breakdown event is "critical"
        DeleteEvent<Breakdown>(x => x.IsCritical);

        // Alternatively, delete the aggregate if the trip
        // is currently in New Mexico and the breakdown is critical
        DeleteEvent<Breakdown>((trip, e) => e.IsCritical && trip.State == "New Mexico");

        DeleteEventAsync<Breakdown>(async (session, trip, e) =>
        {
            var anyRepairShopsInState = await session.Query<RepairShop>()
                .Where(x => x.State == trip.State)
                .AnyAsync();

            // Delete the trip if there are no repair shops in
            // the current state
            return !anyRepairShopsInState;
        });
    }
}

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Another option is to use a method convention with a method named ShouldDelete(), with this equivalent using the ShouldDelete() : bool method convention:

public class TripAggregation: SingleStreamAggregation<Trip>
{
    // The current Trip aggregate would be deleted if
    // the Breakdown event is "critical"
    public bool ShouldDelete(Breakdown breakdown) => breakdown.IsCritical;

    // Alternatively, delete the aggregate if the trip
    // is currently in New Mexico and the breakdown is critical
    public bool ShouldDelete(Trip trip, Breakdown breakdown)
        => breakdown.IsCritical && trip.State == "New Mexico";

    public async Task<bool> ShouldDelete(IQuerySession session, Trip trip, Breakdown breakdown)
    {
        var anyRepairShopsInState = await session.Query<RepairShop>()
            .Where(x => x.State == trip.State)
            .AnyAsync();

        // Delete the trip if there are no repair shops in
        // the current state
        return !anyRepairShopsInState;
    }
}

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The ShouldDelete() method can take any combination of these arguments:

  1. The actual event type
  2. Event<T> where the T is the actual event type. Use this if you want access to the event metadata like versions or timestamps.
  3. IQuerySession if you need to do additional data lookups
  4. The aggregate type

Additionally, ShouldDelete() methods should return either a Boolean or Task<Boolean> if doing data lookups with IQuerySession -- and we'very strongly recommend using strictly asynchronous APIs if running the projection asynchronously or using SaveChangesAsync() when executing projections inline.

"Self-Aggregates"

You can use the AggregateProjection<T> method conventions with a self-aggregate, which we just mean to be an aggregate document type that implements its own Apply() or ShouldDelete() methods to mutate itself. Using that concept, let's take the TripProjection we have been using and apply that instead to a self-aggregating Trip type:

public class Trip
{
    // Probably safest to have an empty, default
    // constructor unless you can guarantee that
    // a certain event type will always be first in
    // the event stream
    public Trip()
    {
    }

    // Create a new aggregate based on the initial
    // event type
    internal Trip(TripStarted started)
    {
        StartedOn = started.Day;
        Active = true;
    }

    public Guid Id { get; set; }
    public int EndedOn { get; set; }

    public double Traveled { get; set; }

    public string State { get; set; }

    public bool Active { get; set; }

    public int StartedOn { get; set; }
    public Guid? RepairShopId { get; set; }

    // The Apply() methods would mutate the aggregate state
    internal void Apply(Arrival e) => State = e.State;
    internal void Apply(Travel e) => Traveled += e.TotalDistance();
    internal void Apply(TripEnded e)
    {
        Active = false;
        EndedOn = e.Day;
    }

    // We think self-aggregates are mostly useful for live aggregations,
    // but hey, if you want to use a self aggregate as an asynchronous projection,
    // you can also specify when the aggregate document should be deleted
    internal bool ShouldDelete(TripAborted e) => true;
    internal bool ShouldDelete(Breakdown e) => e.IsCritical;
    internal bool ShouldDelete(VacationOver e) => Traveled > 1000;
}

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Here's an example of using the Trip self-aggregate:

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

        // Run the Trip as an inline projection
        opts.Projections.SelfAggregate<Trip>(ProjectionLifecycle.Inline);

        // Or run it as an asynchronous projection
        opts.Projections.SelfAggregate<Trip>(ProjectionLifecycle.Async);
    });

    // Or more likely, use it as a live aggregation:

    // Just pretend you already have the id of an existing
    // trip event stream id here...
    var tripId = Guid.NewGuid();

    // We'll open a read only query session...
    using var session = store.QuerySession();

    // And do a live aggregation of the Trip stream
    var trip = await session.Events.AggregateStreamAsync<Trip>(tripId);
}

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