Hot to make aggregate query with .Lookup()? (MongoDB driver, c#)

Question

I'm kind of new with MongoDB driver, so I'm having a bit of confusion regarding queries.

I have collections with the following classes:

[BsonCollection("alerts")]
public class Alert
{
    public ObjectId Id { get; set; }

    public string Name { get; set; }

    public string Description { get; set; }

    [BsonRepresentation(BsonType.ObjectId)]
    public string AlertTypeId { get; set; }

    [BsonIgnore]
    public AlertType AlertType { get; set; }
}

[BsonCollection("alert_types")]
public class AlertType
{
    public ObjectId Id { get; set; }

    public string Name { get; set; }
}

I'm trying to populate the property AlertType in Alert, using Lookup operator. I tried the following:

        var collection = database.GetCollection<Alert>(GetCollectionName(typeof(Alert)));

        var query = collection.Aggregate()
            .Lookup("alert_types", "AlertTypeId", "Id", "AlertType")
            .Unwind("AlertType")
            .As<Alert>()
            .ToList();

But this returns an empty List.

I also tried it this way:

        var query = collection.Aggregate()
            .Lookup("alert_types", x => ObjectId.Parse(x.AlertTypeId), y => y.Id, "AlertType")
            .Unwind("AlertType")
            .As<Alert>()
            .ToList();

But I get the error in Lookup markup:

"The type arguments for method 'IAggregateFluent.Lookup<TForeignDocument, TNewResult>(string, FieldDefinition, FieldDefinition, FieldDefinition, AggregateLookupOptions<TForeignDocument, TNewResult>)' cannot be inferred from the usage. Try specifying the type arguments explicitly."

Moreover, tried with collections as Queryable in this way:

        IEnumerable<Alert> query = from a in collection.AsQueryable()
                                          join o in typesCollection.AsQueryable() on a.AlertTypeId equals o.Id.ToString() into joinedAlertTypes
                                          select new Alert(a, joinedAlertTypes.FirstOrDefault());

(Implementing, as you can see, a constructor in Alert which assigns all its properties and also the AlertType from the join)

But I get error:

Unable to determine the serialization information for the inner key selector in the tree: aggregate([]).GroupJoin(aggregate([]), a => a.AlertTypeId, o => o.Id.ToString(), (a, joinedAlertTypes) => new <>f__AnonymousType0`2(a = a, joinedAlertTypes = joinedAlertTypes))

What am I doing wrong?

Answer 1

Check here: https://github.com/mongodb/mongo-csharp-driver/blob/010e7ee46b085cdd3762894ece9e2d258b66ab0d/tests/MongoDB.Driver.Tests/AggregateFluentTests.cs#L464

The type arguments for method... cannot be inferred from the usage

you should specify the generic arguments explicitly.