c# mongo 2.0 reduce traffic of FindAsync

Question

I have to get some minor data from each document I have in the database but I still want to reduce traffic to prevent "Table-Scan" (just the term, i know its not tables).

I have a collection of lets say "Books" (just because everyone are using it to give examples with ), now, my issue is that I want only the books titles with given author.

var filter = Builders<Book>.Filter.Eq(n => n.Author, AuthorId);

            List<string> books = new List<string>();

            using (var cursor = await BooksCollection.FindAsync(filter))
            {
                while (await cursor.MoveNextAsync())
                {
                    var batch = cursor.Current;
                    foreach (Book b in batch)
                        books.Add(b.Title);
                }
            }

But, when I scan the entire collection result, I'm using big chunks of data, isn't it? lets assume those are not books but entire grid networks and each document is around 5-10 MB and I have thousands of them..how can I reduce the traffic here, without storing this data I need in another collection?

Edit I think its called "Views" in SQL database.

Answer 1

You can reduce the size of the returned documents via projection which you can set in the FindOptions parameter of FindAsync to only include the fields you need:

var filter = Builders<Book>.Filter.Eq(n => n.Author, AuthorId);
// Just project the Title and Author properties of each Book document
var projection = Builders<Book>.Projection
    .Include(b => b.Title)
    .Include(b => b.Author)
    .Exclude("_id"); // _id is special and needs to be explicitly excluded if not needed
var options = new FindOptions<Book, BsonDocument> { Projection = projection };

List<string> books = new List<string>();

using (var cursor = await BooksCollection.FindAsync(filter, options))
{
    while (await cursor.MoveNextAsync())
    {
        var batch = cursor.Current;
        foreach (BsonDocument b in batch)
            // Get the string value of the Title field of the BsonDocument
            books.Add(b["Title"].AsString);
    }
}

Note that the returned documents are BsonDocument objects instead of Book objects as they only contain the projected fields.