Need populate methods such that I can show second-level detail for each top-level item

[Michael Murray]

Hi all - I'm a bit new to mongoose so there may be a straightforward answer to this but I haven't yet found it...

There is a full gist of my schemas/models here: https://gist.github.com/FreeAssoc/10441656

But in short, I have three models -- Post, Opinion and Comment -- that are related thusly:

Post is a top-level "statement".  It has 0-n Opinions about that statement, structured as an array of ObjectIds on the Opinion model 

var PostSchema = new Schema({
(...)
    opinions: [ { type: Schema.ObjectId, ref: 'Opinion' } ]
});

while Opinions have 0-n Comments about each Opinion:

var OpinionSchema = new Schema({
(...)
    comments: [ { type: Schema.ObjectId, ref: 'Comment' } ]
});

When I view each post I want to populate such that I include all opinions and all comments on opinions with each post, and for various business reasons I want to keep posts, opinions and comments in separate (not embedded) models.

What works:
Post.user info is included through ObjectId (works)
detail for each of Post.opinions is included through ObjectIds (works)
What I can't figure out:
detail for post.opinions[n].user is shown for each opinion.user (doesn't yet work)
detail for post.opinion[n].comments -- including the comment.user -- is shown (dosn't yet work)
so in short my issue is that I need to show detail from sub-subdocuments on Post through .populate :).
Advice/pointers welcome.
Thanks - Michael