MartenEventStoreTests.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection.Metadata;
using System.Threading.Tasks;
using Marten;
using Marten.Events.Projections;
using Xunit;
using Xunit.Abstractions;

namespace MartenEventSourceTests;

public class MartenEventStoreTests
{
    private readonly ITestOutputHelper _output;
    private readonly DocumentStore _theStore;
    private Guid _tenancyId = new Guid("00000000-0000-0000-0000-000000000001");
    private Guid _landlordId = new Guid("00000000-0000-0000-0000-000000000002");

    public MartenEventStoreTests(ITestOutputHelper output)
    {
        _output = output;
        _theStore = DocumentStore.For(opts =>
        {
            opts.Connection("Host=localhost;Port=5433;Database=martentest;Username=postgres;Password=*****");
            opts.Projections.SelfAggregate<Tenancy>(ProjectionLifecycle.Inline);
        });
    }

    [Fact]
    public async Task ClearDatabase()
    {
        await _theStore.Advanced.ResetAllData();
    }

    [Fact]
    public async Task CreateNewTenancy()
    {
        var session = _theStore.OpenSession();
        session.Events.StartStream<Tenancy>(_tenancyId, new NewTenancy(DateTime.Today.AddDays(-3), "1 High Street"));
        await session.SaveChangesAsync();
    }

    [Fact]
    public async Task InviteLandlord()
    {
        var session = _theStore.OpenSession();
        var stream = await session.Events.FetchForWriting<Tenancy>(_tenancyId);
        stream.AppendOne(new LandlordAdded(_landlordId));
        await session.SaveChangesAsync();
    }

    [Fact]
    public async Task GetTenancy()
    {
        var session = _theStore.LightweightSession();
        var tenancy = session.Load<Tenancy>(_tenancyId);
        
        _output.WriteLine("Tenancy Start Date: " + tenancy.StartDate);
        _output.WriteLine("Tenancy Address: " + tenancy.PropertyAddress);
        _output.WriteLine("Number of LLs: " + tenancy.LandlordIds.Count());
    }
}

public record NewTenancy(DateTime StartDate, string PropertyAddress);

public record LandlordAdded(Guid LandlordId);


public class Tenancy
{
    public Guid Id { get; private set; }
    public DateTime StartDate { get; private set; }
    public string PropertyAddress { get; private set; }

    public IEnumerable<Guid> LandlordIds { get; private set; } = new List<Guid>();

    public static async Task<Tenancy> Create(NewTenancy @event, IQuerySession session)
    {
        var t = new Tenancy();
        t.StartDate = @event.StartDate;
        t.PropertyAddress = @event.PropertyAddress;
        return t;
    }

    public void Apply(LandlordAdded @event)
    {
        ((List<Guid>)LandlordIds).Add(@event.LandlordId);
    }
}

MartenEventStoreTests.csproj

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <TargetFramework>net6.0</TargetFramework>
        <Nullable>enable</Nullable>

        <IsPackable>false</IsPackable>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="Marten" Version="5.8.0" />
        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
        <PackageReference Include="xunit" Version="2.4.1" />
        <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3">
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
            <PrivateAssets>all</PrivateAssets>
        </PackageReference>
        <PackageReference Include="coverlet.collector" Version="3.1.0">
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
            <PrivateAssets>all</PrivateAssets>
        </PackageReference>
    </ItemGroup>

</Project>

Completely throwaway code to check understanding.

Repro details:
1 - Run ClearDatabase() and then CreateNewTenancy() unit tests. This will create a new event stream, and update the SelfAggregate projection.

2 - select * from mt_doc_tenancy; will then return:

3 - Now run InviteLandlord unit test, which creates a new event on the same stream.

4 - select * from mt_doc_tenancy; will then return:

So the projection has been updated with a Landlord ID, but the start date/property address (set by the first event) have been reset to default. It's like the projection is only applying the latest event.