Mongoose populate sub-sub document

As a complete example calling populate on the result objects:

Item.find({}).populate("comments").exec(function(err,data) {
    if (err) return handleError(err);

    async.forEach(data,function(item,callback) {
        User.populate(item.comments,{ "path": "user" },function(err,output) {
            if (err) throw err; // or do something

            callback();
        });
    }, function(err) {
        res.json(data);
    });

});

The call to .populate() in the form invoked from the model takes either a document or an array as it's first argument. So you loop through the returned results for each item and call populate this way on each "comments" array. The "path" tells the function what it is matching.

This is done using the "async" version of forEach so it is non-blocking, but generally after all the manipulation all of the items in the response are not only populated with comments but the comments themselves have the related "user" details.

Simpler

Item
  .find({})
  .populate({
    path: 'comments.user',
    model: 'users' }
  })
  .exec(function(err, data){
    if (err) return handleError(err);
    res.json(data);
});

One more way (easier) to do this:

Item
  .find({})
  .populate({
	path:     'comments',			
	populate: { path:  'user',
		    model: 'users' }
  })
  .exec(function(err, data){
    if (err) return handleError(err);
    res.json(data);
});