mongodb .net driver V2: How to do array operations

Question

I'm totally lost on doing CRUD and other operations on array elements in an embedded Array in mongodb using c# drivers.

given I have the following classes (simple example):

public class Child
{
    public ObjectId Id;
    public DateTime dateOfBirth;
    public string givenName;
}


class Family
{
    public ObjectId Id;
    public string name;
    public List<Child> children;
}

My collection should store Family documents.

How do I:

1. Add a new child to a Family
2. Delete a certain child
3. Update one Child
4. Count the children of one family
5. get the youngest child of a family
6. Load one specific Child

WITHOUT loading the whole Family object

Although I'm taking part in a mongo university class mongo.net I'm completely lost and the documentation on working with arrays is almost not existing.

I know got answers for 1-4:

    //Add child
    families.UpdateOne(Builders<Family>.Filter.Where(x=>x.name=="Burkhart"), Builders<Family>.Update.AddToSet("children",
        new Child() {dateOfBirth = new DateTime(2005, 4, 26), givenName = "Finn"}));

    // Add another
    families.UpdateOne(Builders<Family>.Filter.Where(x => x.name == "Burkhart"), Builders<Family>.Update.AddToSet("children",
        new Child() { dateOfBirth = new DateTime(2007, 4, 26), givenName = "Florentina" }));

    //remove one
    families.UpdateOne(Builders<Family>.Filter.Where(x => x.name == "Burkhart"),
        Builders<Family>.Update.PullFilter(c => c.children, m => m.givenName == "Florentina"));

    //update one
    families.UpdateOne(Builders<Family>.Filter.Where(x => x.name == "Burkhart" && x.children.Any(c => c.givenName =="Finn")), 
                       Builders<Family>.Update.Set(x=> x.children[-1].givenName,"Finn Linus"));

    //count children
    var numberOfChildren =
        families.Aggregate()
            .Match(f => f.name == "Burkhart")
            .Project(new BsonDocument("count", new BsonDocument("$size", "$children")))
            .FirstOrDefault()
            .GetValue("count")
            .ToInt32();

Answer 1

as 1-4 was fixed by you - congratulations! Please find 5 and 6 as discussed in chat.

Full code in this GitHub repository

        var f = GenerateFaimly();
        collection.InsertOne(f);

        var sort = BsonDocument.Parse("{\"kids.dateOfBirth\": -1}"); // get the youngest 
        var project =
            BsonDocument.Parse(
                "{_id:'$children._id', dateOfBirth:'$children.dateOfBirth', givenName:'$children.givenName', IsAlive:'$children.IsAlive'}");
        var aggregate = collection.Aggregate().Match(x => x.Id == f.Id)

            // .Project(x => new { kids = x.children })
            .Unwind("children").Sort(sort).Limit(1).Project<Child>(project);

        Console.WriteLine(aggregate.ToString());

        var result = aggregate.FirstOrDefault();

        var pojectionIsAlive =
            BsonDocument.Parse(
                "{_id:1, name:1, children:{$filter:{ input:'$children', as:'kids', cond:{$eq:['$$kids.IsAlive', true]}}}}");

        var kids = collection.Aggregate().Match(x => x.Id == f.Id).Project<Family>(pojectionIsAlive).ToList();