Question

Let’s imagine a usual blog engine (just for example). The model would consist of Posts collection with embedded Comments “collection”.

Now, I need to get only 10 recent comments along with my Post data.

1. What’s the best way of doing this?
2. Is this a valuable optimization? (apart from reducing network traffic)

P.S. I use official C# driver + fluent-mongo but I can give up linq for a good cause.

Answer 1

Couldn’t you use the Slice command for retrieving a subset of the array (the last 10)?
Something like:

db.posts.find({}, {comments:{$slice: -10}})

I found this on the official documentation when I had to do something similar.

Link: http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields#RetrievingaSubsetofFields-RetrievingaSubrangeofArrayElements

The easiest way I could find to use the slice command with C# is:

var your_query; 
var slice = Fields.Slice("comments", -10); 
var cursor = collection.Find(your_query).SetFields(slice); 
foreach (var document in cursor) { 
    ...
}