I am trying to add an index to fields which are references to other Schemas. For example my model looks like the following:

var Book = new Schema({
        "title": String,
        "createdAt": Date,
        "publisher": {
             "type": Schema.ObjectId,
             "ref": "Publisher"
        },
        "author": {
             "type": Schema.ObjectId,
             "ref": "Author"
        },
        "isbn": String
    });

So something like the following indexing doesn't work when you perform a search query as described below:

Indexing: Book.index({"title": "text", "publisher.name": "text", "author.firstName": "text"});

Search query:

function searchBooks(req, res) {
    var query = req.query.searchQuery;

    Book.find({ $text: { $search: query } })
        .populate('author publisher')
        .limit(25)
        .exec(function(err, books) {
            if (err) {
                res.json(err);
            } else {
                res.json(books);
            }
        }
    );
}

Does anyone have any suggestions of how to add a text index in for the "publisher" and "author" fields, I am using the "populate" mongodb method to pull in the data for these schemas.