Event Store -- EF Core Configuration

Core Principles

• The event store is an append-only table of Event entities
• A single Event base table uses TPH (table-per-hierarchy) with a discriminator on EventType
• EventConfiguration sets up the discriminator mapping from EventType enum to concrete event classes
• GenericEventConfiguration<TEntity, TData> handles the Newtonsoft.Json conversion for the Data column
• A unique index on (AggregateId, Sequence) prevents duplicate events
• OutboxMessage has a 1:1 relationship with Event via shared primary key (HasForeignKey<OutboxMessage>(e => e.Id))
• The Type column is stored as a string with HasConversion<string>()

Key Patterns

GenericEventConfiguration

Handles JSON serialization of the typed Data property for each concrete event type. Uses Newtonsoft.Json (not System.Text.Json).

using {Company}.{Domain}.Commands.Domain.Events;
using {Company}.{Domain}.Commands.Domain.Events.DataTypes;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;
using Newtonsoft.Json;

namespace {Company}.{Domain}.Commands.Infra.Persistence.Configurations;

public class GenericEventConfiguration<TEntity, TData> : IEntityTypeConfiguration<TEntity>
        where TEntity : Event<TData>
        where TData : IEventData
{
    public void Configure(EntityTypeBuilder<TEntity> builder)
    {
        var jsonSerializerSettings = new JsonSerializerSettings
        {
            NullValueHandling = NullValueHandling.Ignore
        };

        jsonSerializerSettings.Converters.Add(new Newtonsoft.Json.Converters.StringEnumConverter());

        builder.Property(e => e.Data).HasConversion(
             v => JsonConvert.SerializeObject(v, jsonSerializerSettings),
             v => JsonConvert.DeserializeObject<TData>(v)!
        ).HasColumnName("Data");
    }
}

Key details:

• Generic over TEntity (the concrete event class) and TData (the event data type)
• Constraint: TEntity : Event<TData> and TData : IEventData
• Serialization uses NullValueHandling.Ignore and StringEnumConverter
• Deserialization does NOT use the custom settings (just default DeserializeObject)
• Column is named "Data" explicitly

EventConfiguration (Discriminator Mapping)

Maps the EventType enum to concrete event classes using EF Core's discriminator pattern.

using {Company}.{Domain}.Commands.Domain.Enums;
using {Company}.{Domain}.Commands.Domain.Events;
using {Company}.{Domain}.Commands.Domain.Events.Orders;
using {Company}.{Domain}.Commands.Domain.Events.Invoices;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;

namespace {Company}.{Domain}.Commands.Infra.Persistence.Configurations;

public class EventConfiguration : IEntityTypeConfiguration<Event>
{
    public void Configure(EntityTypeBuilder<Event> builder)
    {
        builder.HasIndex(e => new { e.AggregateId, e.Sequence }).IsUnique();

        builder.Property(e => e.Type)
               .HasMaxLength(128)
               .HasConversion<string>();

        builder.HasDiscriminator(e => e.Type)
            .HasValue<OrderCreated>(EventType.OrderCreated)
            .HasValue<OrderUpdated>(EventType.OrderUpdated)
            .HasValue<OrderItemsAdded>(EventType.OrderItemsAdded)
            .HasValue<OrderItemsRemoved>(EventType.OrderItemsRemoved)
            .HasValue<InvoiceGenerated>(EventType.InvoiceGenerated)
            .HasValue<InvoiceUpdated>(EventType.InvoiceUpdated);
    }
}

Key details:

• Unique composite index on (AggregateId, Sequence) prevents duplicate events
• Type is stored as a string via HasConversion<string>() with max length 128
• The discriminator is the EventType enum, mapping each enum value to a concrete event class
• Every new event type must be registered here

OutboxMessageConfiguration

using {Company}.{Domain}.Commands.Domain.Entities;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;

namespace {Company}.{Domain}.Commands.Infra.Persistence.Configurations;

public class OutboxMessageConfiguration : IEntityTypeConfiguration<OutboxMessage>
{
    public void Configure(EntityTypeBuilder<OutboxMessage> builder)
    {
        builder.HasOne(e => e.Event)
            .WithOne()
            .HasForeignKey<OutboxMessage>(e => e.Id)
            .IsRequired()
            .OnDelete(DeleteBehavior.Cascade);
    }
}

Key details:

• 1:1 relationship: OutboxMessage.Id IS the foreign key to Event.Id (shared primary key)
• HasOne(e => e.Event).WithOne() -- OutboxMessage has an Event navigation property
• HasForeignKey<OutboxMessage>(e => e.Id) -- the OutboxMessage's own Id is the FK
• Cascade delete: when an OutboxMessage is deleted, the relationship is cleaned up

ApplicationDbContext

using {Company}.{Domain}.Commands.Domain.Entities;
using {Company}.{Domain}.Commands.Domain.Events;
using {Company}.{Domain}.Commands.Domain.Events.DataTypes;
using {Company}.{Domain}.Commands.Domain.Events.Orders;
using {Company}.{Domain}.Commands.Domain.Events.Invoices;
using {Company}.{Domain}.Commands.Infra.Persistence.Configurations;
using Microsoft.EntityFrameworkCore;

namespace {Company}.{Domain}.Commands.Infra.Persistence;

public class ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : DbContext(options)
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Base configurations
        modelBuilder.ApplyConfiguration(new EventConfiguration());
        modelBuilder.ApplyConfiguration(new OutboxMessageConfiguration());

        // GenericEventConfiguration for each concrete event type
        modelBuilder.ApplyConfiguration(new GenericEventConfiguration<OrderCreated, OrderCreatedData>());
        modelBuilder.ApplyConfiguration(new GenericEventConfiguration<OrderUpdated, OrderUpdatedData>());
        modelBuilder.ApplyConfiguration(new GenericEventConfiguration<OrderItemsAdded, OrderItemsAddedData>());
        modelBuilder.ApplyConfiguration(new GenericEventConfiguration<OrderItemsRemoved, OrderItemsRemovedData>());
        modelBuilder.ApplyConfiguration(new GenericEventConfiguration<InvoiceGenerated, InvoiceGeneratedData>());
        modelBuilder.ApplyConfiguration(new GenericEventConfiguration<InvoiceUpdated, InvoiceUpdatedData>());

        base.OnModelCreating(modelBuilder);
    }

    public DbSet<Event> Events { get; set; }
    public DbSet<OutboxMessage> OutboxMessages { get; set; }
}

Key details:

• Single DbSet<Event> for all events (TPH pattern)
• Each concrete event type needs BOTH a discriminator entry in EventConfiguration AND a GenericEventConfiguration registration
• base.OnModelCreating(modelBuilder) is called at the end
• DbSets use { get; set; } (not expression-bodied => Set<T>())

Event Repository

namespace {Company}.{Domain}.Commands.Infra.Persistence.Repositories;

public class EventRepository : AsyncRepository<Event>, IEventRepository
{
    private readonly ApplicationDbContext _appDbContext;

    public EventRepository(ApplicationDbContext appDbContext) : base(appDbContext)
    {
        _appDbContext = appDbContext;
    }

    public async Task<IEnumerable<Event>> GetAllByAggregateIdAsync(
        Guid aggregateId, CancellationToken cancellationToken)
        => await _appDbContext.Events
                              .AsNoTracking()
                              .Where(e => e.AggregateId == aggregateId)
                              .OrderBy(e => e.Sequence)
                              .ToListAsync(cancellationToken);
}

Anti-Patterns

Anti-Pattern	Correct Approach
Updating existing events	Events are immutable -- append only
Missing discriminator mapping	Every concrete event needs HasValue<> in EventConfiguration
Missing GenericEventConfiguration	Every concrete event also needs its Data JSON conversion registered
Using System.Text.Json for Data column	Use Newtonsoft.Json (project convention)
Separate tables per event type	Use TPH with single Events table and discriminator
OutboxMessage with its own independent Id	OutboxMessage.Id IS the Event.Id (shared PK/FK)

Detect Existing Patterns

# Find ApplicationDbContext
grep -r "class ApplicationDbContext" --include="*.cs" src/

# Find GenericEventConfiguration registrations
grep -r "GenericEventConfiguration" --include="*.cs" src/

# Find discriminator setup
grep -r "HasDiscriminator" --include="*.cs" src/

# Find EventConfiguration
grep -r "class EventConfiguration" --include="*.cs" src/

# Find OutboxMessageConfiguration
grep -r "OutboxMessageConfiguration" --include="*.cs" src/

Adding to Existing Project

1. Add new EventType value to the enum
2. Create concrete event class and event data record
3. Add discriminator mapping in EventConfiguration (HasValue<NewEvent>(EventType.NewEvent))
4. Add GenericEventConfiguration registration in ApplicationDbContext.OnModelCreating
5. Add EF Core migration after registering new event configurations
6. Verify the unique index on (AggregateId, Sequence) exists