Question

Lets say we have the following document structure:

class BlogPost
{
   [MongoIdentifier]
   public Guid Id{get;set;}
   public string Body{get;set;}
   ....
}

class Comment
{
   [MongoIdentifier]
   public Guid Id{get;set;}
   public string Body {get;set;}
}

If we assume that multiple users may post comments for the same post, what would be the best way to model the relation between these?

if Post have a collection of comments, I might get concurrency problems, won’t I ?

And placing a FK like attribute on Comment seems too relational , or?

Answer 1

You basically have two options: 1. Aggregate comments in the post document, or 2. Model post and comment as documents.

If you aggregate the comments, you should either a) implement a revision number on the post, allowing you to detect race conditions and implement handling of optimistic concurreny, or b) add new comments with a MongoDB modifier – e.g. something like

var posts = mongo.GetCollection<Post>();
var crit = new { Id = postId };
var mod = new { Comments = M.Push(new Comment(...)) };

posts.Update(crit, mod, false, false);

If you model post and comment as separate documents, handling concurrency is probably easier, but you lose the ability to load a post and its comments with a single findOne command.

In my opinion, (1) is by far the most interesting option because it models the post as an aggregate object, which is exactly what it is when you put your OO glasses on :). It’s definitely the document-oriented approach, whereas (2) resembles the flat structure of a relational database.