How to count, select and update nested elements in mongodb + C#

I am creating a open source social network (I don't know if I can post the URL here) using .net core 2.1, mongo atlas using the driver v2.9.2

I am migrating from MS SQL (which I know how to do stuff on it) to mongo (which I am still learning).

I have two entities, UserContent and UserContentComment, both inheriting from Entity:

public class Entity
{
    public Guid Id { get; set; }

    public Guid UserId { get; set; }

    public DateTime CreateDate { get; set; }

    public Entity()
    {
        CreateDate = DateTime.Now;
    }
}

public class UserContent : Entity
{
    public string AuthorName { get; set; }

    public string Content { get; set; }

    public virtual List<UserContentComment> Comments { get; set; }
}

public class UserContentComment : Entity
{
    public Guid UserContentId { get; set; }

    public string AuthorName { get; set; }

    public string Text { get; set; }
}

I will probably clean some properties since I won't need them anymore like UserContentId, for example, because I will add UserContentComments as a nested array "Comments" of UserContent like this:

{
    "_id" : UUID("ff6e86cc-bddf-419d-e049-08d6b903e6db"),
    "_t" : [
        "Entity",
        "UserContent"
    ],
    "userId" : UUID("9d6e26ca-5bc7-4121-9b03-ed7f9a442cf6"),
    "createDate" : ISODate("2019-04-04T16:38:23.740-03:00"),
    "authorName" : "User Full Name",
    "content" : "content main text",
    "comments" : [
        {
            "_t" : [
                "Entity",
                "UserContentComment"
            ],
            "_id" : UUID("d7dc1328-b1bc-4f1f-a9a5-08d6bf05b672"),
            "userId" : UUID("5f9eeb34-22e2-43bf-9130-034a7997d2af"),
            "createDate" : ISODate("2019-04-12T05:15:20.636-03:00"),
            "userContentId" : UUID("ff6e86cc-bddf-419d-e049-08d6b903e6db"),
            "authorName" : "User Full Name",
            "text" : "Hello from Russia!"
        }
    ]
}

My problem is I need to:

• Count comments BY UserId;
• Get comments BY UserId;
• Get single comment BY Id;
• Update single comment BY Id;
• Update all comments BY UserId;

How can I achieve that using the mongodb driver for C#?

Thank you very much for the help!

UPDATE - What I got so far

To count comments, I got this three ways:

    var count = DbSet.Aggregate()
                .Match(x => x.Comments.Any(y => y.UserId == userId))
                .Project(new BsonDocument("count", new BsonDocument("$size", "$comments"))).ToList();

    var q = Builders<UserContent>.Filter.ElemMatch(x => x.Comments, d => d.UserId == userId);
    var p = Builders<UserContent>.Projection.ElemMatch(x => x.Comments, d => d.UserId == userId);

    var data = DbSet.Find(q).Project(p).ToList();
    var data2 = DbSet.Find(q).ToList();

These three different queries, return 28, because the user really commented on 28 posts, but there are a total of 31 comments on the database because the user commented multiple times on some posts. What am I missing here?

UPDATE now the COUNT works

I got the count working using this code:

    DbSet.AsQueryable().SelectMany(x => x.Comments).Where(where).Count();

I got 31 using the above code, now is correct!

The question now is: will this run on memory or on DB?