IFilteredMongoCollection<T> Ignores Type Filtering in Queries in Driver 3.x

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    • Type: Bug
    • Resolution: Works as Designed
    • Priority: Unknown
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    • Affects Version/s: None
    • Component/s: None
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    • Dotnet Drivers
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      1. What would you like to communicate to the user about this feature? 2. Would you like the user to see examples of the syntax and/or executable code and its output? 3. Which versions of the driver/connector does this apply to?
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      Summary

      We’ve encountered a regression in MongoDB.Driver 3.x when using IFilteredMongoCollection<T>. Specifically, when calling .OfType<T>() on a collection and subsequently executing a query, the expected filtering on _t is missing, also when using HierarchicalDiscriminatorConvention.

      This behavior differs from MongoDB.Driver 2.x, where the generated query correctly includes _t for filtering by type.

      How to Reproduce

      using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using MongoDB.Driver.Core.Events;

BsonClassMap.RegisterClassMap<Document>();
BsonClassMap.RegisterClassMap<Animal>();
BsonClassMap.RegisterClassMap<Human>();
BsonClassMap.RegisterClassMap<Hawk>();

var connectionString = "mongodb://localhost/db?directConnection=true";
var settings = MongoClientSettings.FromUrl(new MongoUrl(connectionString));

settings.ClusterConfigurator = builder =>
{
    builder.Subscribe<CommandStartedEvent>(e =>
    {
        Console.WriteLine($"Command {e.CommandName} -> {e.Command.ToJson()}");
    });
};
var client = new MongoClient(settings);

// Bootstrap data
var collection = client.GetDatabase("db").GetCollection<Animal>("animals");
var human = new Human { Legs = 4, Eyes = 2 };
var hawk = new Hawk { Legs = 4, Wings = 2 };
await collection.InsertManyAsync([human, hawk]);

// Query using filtered collection
var filteredCollection = client.GetDatabase("db").GetCollection<Human>("animals").OfType<Human>();
var document = await filteredCollection.Find(Builders<Human>.Filter.Eq(x => x.Legs, 4)).ToListAsync();

// Model definitions
public class Document 
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }
}

[BsonDiscriminator(RootClass = true)]
public class Animal : Document
{
    public int Legs { get; set; }
}

public class Human : Animal
{
    public int Eyes { get; set; }
}

public class Hawk : Animal
{
    public int Wings { get; set; }
}

       

      Observed Behavior:

      Using MongoDB.Driver 2.30.0, the generated query is:

      { "find": "animals", "filter":  { "_t": "Human", "Legs": 4 } }

      However, with MongoDB.Driver 3.2.1, the _t discriminator is missing:

      { "find": "animals", "filter":  { "Legs": 4 } }

      Can you confirm whether this is an intentional change or a regression? If intentional, what is the recommended way to preserve _t filtering in queries when using IFilteredMongoDbCollection?

            Assignee:
            Ferdinando Papale
            Reporter:
            Jarosław Szmigielski
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              Created:
              Updated:
              Resolved: