I am using the MongoDB EF Core provider and I am getting duplicates for embedded collections of complex objects when I load the root object. I have noticed this with latest version 8.0 and previous 7.0.0-preview.1. I am using .net8. This is my .proj file:

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net8.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
     </PropertyGroup>
  
    <ItemGroup>
        <PackageReference Include="MongoDB.EntityFrameworkCore" Version="8.0.0" />
    </ItemGroup>
</Project>

Here is the sample which triggers the bug:

using Microsoft.EntityFrameworkCore;
using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.EntityFrameworkCore.Extensions;

var connectionString = Environment.GetEnvironmentVariable("MONGODB_URL");
var client = new MongoClient(connectionString);
var db = MflixDbContext.Create(client.GetDatabase("sample"));

var movie1 = new Movie();
movie1._id = ObjectId.GenerateNewId();
movie1.title = "Movie 1";
movie1.actors.Add(new Actor() { FirstName = "John", LastName = "Smith" });
movie1.actors.Add(new Actor() { FirstName = "Mary", LastName = "Smith" });
db.Movies.Add(movie1);
Console.WriteLine("Total actors going to the database: " + movie1.actors.Count());
db.SaveChanges();

var movieFromDb = db.Movies.FirstOrDefault(x => x._id == movie1._id);
Console.WriteLine("Movie: " + movieFromDb.title);
Console.WriteLine("Total actors after loading from database: " + movieFromDb.actors.Count());

internal class MflixDbContext : DbContext
{
    public DbSet<Movie> Movies { get; init; }
    public static MflixDbContext Create(IMongoDatabase database) =>
        new(new DbContextOptionsBuilder<MflixDbContext>()
            .UseMongoDB(database.Client, database.DatabaseNamespace.DatabaseName)
            .Options);
    public MflixDbContext(DbContextOptions options)
        : base(options) {}

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<Movie>().ToCollection("movies");
    }
}

internal class Actor
{
    public string FirstName { get; set;}
    public string LastName { get; set;}
}

internal class Movie
{
    public ObjectId _id { get; set; }
    public string title { get; set; }
    public List<Actor> actors { get; set; } = new List<Actor>();
}

If you run, you will get the following output:

Total actors going to the database: 2
Movie: Movie 1
Total actors after loading from database: 4

If you look at the “movies” collection in mongodb, you will find no duplicates:

sample> db.movies.find()
[
  {
    _id: ObjectId('66396aa29571e499f39d6fab'),
    title: 'Movie 1',
    actors: [
      { FirstName: 'John', LastName: 'Smith' },
      { FirstName: 'Mary', LastName: 'Smith' }
    ]
  }
]

I hope I am doing something wrong because this is a pretty big bug to be released. While 7.0.0 was preview release, 8.0 is not a preview release but a general release.

I have also noticed situations where only a portion of the children items are duplicated, instead of all of them.

Can anyone confirm they are having this issue? I will post this in the MongoDB forums also.