EF Core Projections

Marten provides first-class support for projecting events into Entity Framework Core DbContext entities. This lets you use EF Core's model configuration, change tracking, and migration tooling while still benefiting from Marten's event sourcing infrastructure.

The Marten.EntityFrameworkCore NuGet package provides three projection base classes:

Base Class	Use Case
EfCoreSingleStreamProjection<TDoc, TId, TDbContext>	Aggregate a single event stream into one EF Core entity
EfCoreMultiStreamProjection<TDoc, TId, TDbContext>	Aggregate events across multiple streams into one EF Core entity
EfCoreEventProjection<TDbContext>	React to individual events, writing to both EF Core and Marten

All three types support Inline, Async, and Live projection lifecycles.

Installation

Add the Marten.EntityFrameworkCore NuGet package to your project:

dotnet add package Marten.EntityFrameworkCore

Defining a DbContext

EF Core projections require a DbContext with entity mappings. Use OnModelCreating to configure table names and column mappings:

public class OrderDbContext : DbContext
{
    public OrderDbContext(DbContextOptions<OrderDbContext> options) : base(options) { }

    public DbSet<Order> Orders => Set<Order>();
    public DbSet<OrderSummary> OrderSummaries => Set<OrderSummary>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>(entity =>
        {
            entity.ToTable("ef_orders");
            entity.HasKey(e => e.Id);
            entity.Property(e => e.Id).HasColumnName("id");
            entity.Property(e => e.CustomerName).HasColumnName("customer_name");
            entity.Property(e => e.TotalAmount).HasColumnName("total_amount");
            entity.Property(e => e.ItemCount).HasColumnName("item_count");
            entity.Property(e => e.IsShipped).HasColumnName("is_shipped");
        });
    }
}

TIP

Entity tables defined in the DbContext are automatically migrated alongside Marten's own schema objects through Weasel. You do not need to run dotnet ef database update separately.

Single Stream Projections

Use EfCoreSingleStreamProjection<TDoc, TId, TDbContext> to build an aggregate from a single event stream and persist it through EF Core.

Entity and Events

// Events
public record OrderPlaced(Guid OrderId, string CustomerName, decimal Amount, int Items);
public record OrderShipped(Guid OrderId);
public record OrderCancelled(Guid OrderId);

// EF Core entity (the aggregate)
public class Order
{
    public Guid Id { get; set; }
    public string CustomerName { get; set; } = string.Empty;
    public decimal TotalAmount { get; set; }
    public int ItemCount { get; set; }
    public bool IsShipped { get; set; }
    public bool IsCancelled { get; set; }
}

Projection Class

Override ApplyEvent to handle each event. The DbContext is available for querying or writing side effects:

public class OrderAggregate
    : EfCoreSingleStreamProjection<Order, Guid, OrderDbContext>
{
    public override Order? ApplyEvent(
        Order? snapshot, Guid identity, IEvent @event,
        OrderDbContext dbContext, IQuerySession session)
    {
        switch (@event.Data)
        {
            case OrderPlaced placed:
                return new Order
                {
                    Id = placed.OrderId,
                    CustomerName = placed.CustomerName,
                    TotalAmount = placed.Amount,
                    ItemCount = placed.Items
                };

            case OrderShipped:
                if (snapshot != null) snapshot.IsShipped = true;
                return snapshot;

            case OrderCancelled:
                if (snapshot != null) snapshot.IsCancelled = true;
                return snapshot;
        }

        return snapshot;
    }
}

Registration

Use the StoreOptions.Add() extension method to register the projection. This sets up EF Core storage, Weasel schema migration, and the projection lifecycle in one call:

var store = DocumentStore.For(opts =>
{
    opts.Connection(connectionString);
    opts.Add(new OrderAggregate(), ProjectionLifecycle.Inline);
});

Multi Stream Projections

Use EfCoreMultiStreamProjection<TDoc, TId, TDbContext> to aggregate events from multiple streams into a single EF Core entity.

Entity and Events

public record CustomerOrderPlaced(Guid OrderId, string CustomerName, decimal Amount);
public record CustomerOrderCompleted(Guid OrderId, string CustomerName);

public class CustomerOrderHistory
{
    public string Id { get; set; } = string.Empty;
    public int TotalOrders { get; set; }
    public decimal TotalSpent { get; set; }
}

Projection Class

Use the constructor to configure event-to-aggregate identity mapping, then override ApplyEvent:

public class CustomerOrderHistoryProjection
    : EfCoreMultiStreamProjection<CustomerOrderHistory, string, OrderDbContext>
{
    public CustomerOrderHistoryProjection()
    {
        // Map events to the aggregate identity (customer name in this case)
        Identity<CustomerOrderPlaced>(e => e.CustomerName);
        Identity<CustomerOrderCompleted>(e => e.CustomerName);
    }

    public override CustomerOrderHistory? ApplyEvent(
        CustomerOrderHistory? snapshot, string identity,
        IEvent @event, OrderDbContext dbContext)
    {
        snapshot ??= new CustomerOrderHistory { Id = identity };

        switch (@event.Data)
        {
            case CustomerOrderPlaced placed:
                snapshot.TotalOrders++;
                snapshot.TotalSpent += placed.Amount;
                break;
        }

        return snapshot;
    }
}

Registration

var store = DocumentStore.For(opts =>
{
    opts.Connection(connectionString);
    opts.Events.StreamIdentity = StreamIdentity.AsString;
    opts.Add(new CustomerOrderHistoryProjection(), ProjectionLifecycle.Async);
});

Event Projections

Use EfCoreEventProjection<TDbContext> when you need to react to individual events and write to both EF Core entities and Marten documents in the same transaction:

Projection Class

public class OrderSummaryProjection : EfCoreEventProjection<OrderDbContext>
{
    protected override async Task ProjectAsync(
        IEvent @event, OrderDbContext dbContext,
        IDocumentOperations operations, CancellationToken token)
    {
        switch (@event.Data)
        {
            case OrderPlaced placed:
                // Write to EF Core
                dbContext.OrderSummaries.Add(new OrderSummary
                {
                    Id = placed.OrderId,
                    CustomerName = placed.CustomerName,
                    TotalAmount = placed.Amount,
                    ItemCount = placed.Items,
                    Status = "Placed"
                });

                // Also write to Marten
                operations.Store(new Order
                {
                    Id = placed.OrderId,
                    CustomerName = placed.CustomerName,
                    TotalAmount = placed.Amount,
                    ItemCount = placed.Items
                });
                break;

            case OrderShipped shipped:
                var summary = await dbContext.OrderSummaries
                    .FindAsync(new object[] { shipped.OrderId }, token);
                if (summary != null)
                {
                    summary.Status = "Shipped";
                }
                break;
        }
    }
}

Registration

EfCoreEventProjection uses the standard Projections.Add() method with a separate call to register entity tables:

var store = DocumentStore.For(opts =>
{
    opts.Connection(connectionString);
    opts.Projections.Add(new OrderSummaryProjection(), ProjectionLifecycle.Inline);
    opts.AddEntityTablesFromDbContext<OrderDbContext>();
});

Conjoined Multi-Tenancy

EF Core single-stream and multi-stream projections support Marten's conjoined multi-tenancy. When the event store uses TenancyStyle.Conjoined, the projection infrastructure automatically writes the tenant ID to each projected entity.

Requirements

Your aggregate entity must implement ITenanted from Marten.Metadata. This interface adds a TenantId property that the projection infrastructure uses to write the tenant identifier:

using Marten.Metadata;

public class TenantedOrder : ITenanted
{
    public Guid Id { get; set; }
    public string CustomerName { get; set; } = string.Empty;
    public decimal TotalAmount { get; set; }
    public int ItemCount { get; set; }
    public bool IsShipped { get; set; }
    public string? TenantId { get; set; } // Required by ITenanted
}

The DbContext must also map the TenantId property to a column:

public class TenantedOrderDbContext : DbContext
{
    public TenantedOrderDbContext(DbContextOptions<TenantedOrderDbContext> options)
        : base(options) { }

    public DbSet<TenantedOrder> TenantedOrders => Set<TenantedOrder>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<TenantedOrder>(entity =>
        {
            entity.ToTable("ef_tenanted_orders");
            entity.HasKey(e => e.Id);
            entity.Property(e => e.Id).HasColumnName("id");
            entity.Property(e => e.CustomerName).HasColumnName("customer_name");
            entity.Property(e => e.TotalAmount).HasColumnName("total_amount");
            entity.Property(e => e.ItemCount).HasColumnName("item_count");
            entity.Property(e => e.IsShipped).HasColumnName("is_shipped");
            entity.Property(e => e.TenantId).HasColumnName("tenant_id");
        });
    }
}

Projection Class

The projection class itself does not need any special tenancy logic. The base infrastructure sets TenantId automatically:

public class TenantedOrderAggregate
    : EfCoreSingleStreamProjection<TenantedOrder, Guid, TenantedOrderDbContext>
{
    public override TenantedOrder? ApplyEvent(
        TenantedOrder? snapshot, Guid identity, IEvent @event,
        TenantedOrderDbContext dbContext, IQuerySession session)
    {
        switch (@event.Data)
        {
            case OrderPlaced placed:
                return new TenantedOrder
                {
                    Id = placed.OrderId,
                    CustomerName = placed.CustomerName,
                    TotalAmount = placed.Amount,
                    ItemCount = placed.Items
                };

            case OrderShipped:
                if (snapshot != null) snapshot.IsShipped = true;
                return snapshot;
        }

        return snapshot;
    }
}

Registration

var store = DocumentStore.For(opts =>
{
    opts.Connection(connectionString);
    opts.Events.TenancyStyle = TenancyStyle.Conjoined;
    opts.Add(new TenantedOrderAggregate(), ProjectionLifecycle.Inline);
});

Appending Events with a Tenant

Use ForTenant() when opening a session to associate events with a specific tenant:

await using var session = store.LightweightSession("tenant-alpha");
session.Events.StartStream(orderId, new OrderPlaced(orderId, "Alice", 100m, 3));
await session.SaveChangesAsync();
// The projected row in ef_tenanted_orders will have tenant_id = 'tenant-alpha'

Validation

Marten validates your configuration at startup. If the event store uses conjoined tenancy but your aggregate type does not implement ITenanted, Marten throws an InvalidProjectionException with a descriptive error message.

Limitations

• EfCoreEventProjection does not support conjoined tenancy validation. The event projection base class (EfCoreEventProjection<TDbContext>) is a lower-level IProjection implementation that does not participate in the aggregate tenancy validation. If you need multi-tenant event projections, you are responsible for reading the tenant ID from @event.TenantId and writing it yourself.

• Multi-stream projections with non-unique keys across tenants. When using EfCoreMultiStreamProjection with conjoined tenancy, be aware that DbContext.FindAsync looks up entities by primary key only, not by a composite of primary key + tenant ID. If two tenants can produce the same aggregate key (e.g., a customer name), you must ensure globally unique aggregate IDs (such as GUIDs) or configure a composite primary key in EF Core that includes the tenant ID column.

Composite Projections

EF Core projections can participate in composite projections for multi-stage processing:

var store = DocumentStore.For(opts =>
{
    opts.Connection(connectionString);
    opts.Projections.Composite(composite =>
    {
        composite.Add(opts, new OrderAggregate(), stageNumber: 1);
        composite.Add(opts, new CustomerOrderHistoryProjection(), stageNumber: 2);
    }, ProjectionLifecycle.Async);
});

DbContext Configuration

All EF Core projection types expose a ConfigureDbContext method you can override to customize the DbContextOptionsBuilder. The Npgsql provider is already configured before this method is called:

public class MyProjection
    : EfCoreSingleStreamProjection<Order, Guid, OrderDbContext>
{
    public override void ConfigureDbContext(
        DbContextOptionsBuilder<OrderDbContext> builder)
    {
        builder.EnableSensitiveDataLogging();
    }
}

How It Works

Under the hood, EF Core projections:

1. Create a per-slice DbContext using the same PostgreSQL connection as the Marten session
2. Register a transaction participant so the DbContext's SaveChangesAsync is called within Marten's transaction, ensuring atomicity
3. Migrate entity tables through Weasel alongside Marten's own schema objects, so dotnet ef migrations are not needed
4. Use EF Core change tracking for insert vs. update detection (detached entities are added; unchanged entities are marked as modified)