mongoose models - trying to determine second-level populate issue

gistfile1.txt

// ... requires elided...
var UserSchema = new Schema({
    name: String,
    email: String,
    username: {
        type: String,
        unique: true
    },
    provider: String,
    hashed_password: String,
    salt: String,
});
mongoose.model('User', UserSchema);

var CommentSchema = new Schema({
    created: {
        type: Date,
        default: Date.now
    },
    modified: {
        type: Date,
        default: Date.now
    },
    user: {
        type: Schema.ObjectId,
        required: true,
        ref: 'User'
    },
    content: {
        type: String,
        default: '',
        trim: true
    }
});
CommentSchema.statics = {
    load: function(id, cb) {
        this.findOne({
            _id: id
        }).populate('user', 'name username').exec(cb);
    }
};
mongoose.model('Comment', CommentSchema);

var OpinionSchema = new Schema({
    created: {
        type: Date,
        default: Date.now
    },
    modified: {
        type: Date,
        default: Date.now
    },
    content: {
        type: String,
        default: '',
        trim: true
    },
    // user for opinion -- required
    user: {
        type: Schema.ObjectId,
        required: true,
        ref: 'User'
    },
    // comments on opinion
    comments: [ { type: Schema.ObjectId, ref: 'Comment' } ]
});

OpinionSchema.statics = {
    load: function(id, cb) {
        this.findOne({
            _id: id
        })
        .populate('user', 'name username')
        .populate('comments')
        .exec(cb);
    }
};
mongoose.model('Opinion', OpinionSchema);

var PostSchema = new Schema({
    created: {
        type: Date,
        default: Date.now
    },
    modified: {
        type: Date,
        default: Date.now
    },
    content: {
        type: String,
        default: '',
        trim: true
    },
    user: {
        type: Schema.ObjectId,
        ref: 'User'
    },
    image_url: {
        type: String,
        default: '',
        trime: true
    },
    opinions: [ { type: Schema.ObjectId, ref: 'Opinion' } ]
});

PostSchema.statics = {
    load: function(id, cb) {
        this.findOne({
            _id: id
        })
        .populate('user opinions')
        .exec(cb);
    }
};
mongoose.model('Post', PostSchema);

the goal here is to populate Post model such that:

• Post.user info is included through ObjectId (works)
• detail for each of Post.opinions is included through ObjectIds (works)
• 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 :).