MongoDB nested sets
mongodb
Solution
I think there is no perfect solution - depends on what operations are more important for your app. I believe Silicon Alley Insider stores comments nested with MongoDB for example. That does make the query you mention harder.
One option is store at top-level in the post a list of all commenters in an array. Think of that as denormalized data. Then one can easily find all posts which involve a certain commenter. Then to drill down, you use map/reduce or db.eval() to get the nested post info within.
One other note - if you are dealing with a single document, db.eval() is probably lighter-weight than map/reduce. $where is also an option but can be slow so I like the additional 'list of commenters' mentioned above - not it is also easy to index that array too (see 'Multikey' in the docs).
See also: http://groups.google.com/group/mongodb-user/browse_thread/thread/df8250573c91f75a/e880d9c57e343b52?lnk=gst&q=trees#e880d9c57e343b52
Problem
What're the best practices to store nested sets (like trees of comments) in MongoDB? I mean, every comment can have a parent comment and children-comments (answers). Storing them like this: ``` { title: "Hello", body: "Please comment me!", comments: [ { author: "Peter", text: "Hi there", answers: [ { author: "Peter", text: "Hi there", answers: [ { author: "Ivan", text: "Hi there" }, { author: "Nicholas", text: "Hi there" } ] }, { author: "Ivan", text: "Hi there" }, { author: "Nicholas", text: "Hi there" }, ] }, { author: "Ivan", text: "Hi there" }, { author: "Nicholas", text: "Hi there" }, ] } ``` is not cool, because we can't, for example, ask for "all post which are commented by Peter" without map/reduce.