Use separate collections

db.posts.find();
{_id: 1, title: 'unicorns are awesome', ...}

db.comments.find();
{_id: 1, post_id: 1, title: 'i agree', ...}
{_id: 2, post_id: 1, title: 'they kill vampires too!', ...}

Use embedded documents

db.posts.find();
{_id: 1, title: 'unicorns are awesome', ..., comments: [
  {title: 'i agree', ...},
  {title: 'they kill vampires too!', ...}
]}

// sort comments however you want
db.comments.find({post_id: 3}).sort({votes: -1}).limit(5)

// pull out one or more specific comment(s)
db.comments.find({post_id: 3, user: 'leto'})

// get all of a user's comments joining the posts to get the title
var comments = db.comments.find({user: 'leto'}, {post_id: true})
var postIds = comments.map(function(c) { return c.post_id; });
db.posts.find({_id: {$in: postIds}}, {title: true});

// you can select a range (useful for paging)
// but can't sort, so you are limited to the insertion order
db.posts.find({_id: 3}, {comments: {$slice: [0, 5]}})

// you can select the post without any comments also
db.posts.find({_id: 54}, {comments: -1})

// you <em>can't</em> use the update's position operator ($) for field selections
db.posts.find({'comments.user': 'leto'}, {title: 1, 'comments.$': 1})

// finding a post + its comments is two queries and requires extra work
// in your code to make it all pretty (or your ODM might do it for you)
db.posts.find({_id: 9001});
db.comments.find({post_id: 9001})

// finding a post + its comments
db.posts.find({_id: 9001});

// separate collection insert and update
db.comments.insert({post_id: 43, title: 'i hate unicrons', user: 'dracula'});
db.comments.update({_id: 4949}, {$set : {title: 'i hate unicorns'}});

// embedded document insert and update
db.posts.update({_id: 43}, {$push: {title: 'lol @ emo vampire', user: 'paul'}})
// this specific update requires that we store an _id with each comment
db.posts.update( {'comments._id': 4949}, {$inc:{'comments.$.votes':1}})