Resolving a ref inside of a MongoDB aggregate - mongodb

Question

I have a Product model object that has the following field in its schema:
category : { type: ObjectId, turnOn: false, ref: "category" }
It references a category model that has a title field in it:
var categorySchema = Schema({
title : { type: String }
});
I'm using the product.category property (which is of type ObjectId as shown above) in a MongoDB aggregate but really want the category.title property from the category model rather than _id in the final resultset.
The following code gets the job done, but you'll see that I'm having to do some looping at the end to "resolve" the title field for the given product.category (ObjectId). Is there anyway to do all of that within the aggregate? In other words, is there a way to get the category model object's title field in the groups that are returned rather than having to do the extra looping work? Based on posts I've researched I don't see a built-in way but wanted to double-check.
getProductsGroupedByCategory = function(callback) {
Category.find(function(err, cats) {
var aggregate = [
{
$group: {
_id: "$category",
products: {
$push: {
title: "$title",
authors: "$authors",
publishDate: "$publishDate",
description: "$description"
}
}
}
},
{
$sort: {
"_id": 1
}
}
];
Product.aggregate(aggregate, function(err, catProducts) {
//Grab name of category and associate with each group
//since we only have the category_id at this point
for (var i = 0; i<catProducts.length;i++) {
var catProduct = catProducts[i];
for (var j=0;j<cats.length;j++) {
if (catProduct._id.toString() === cats[j]._id.toString()) {
catProduct.category = cats[j].title;
}
}
};
callback(err, catProducts);
});
});
}, //more code follows

Answer 1

An example datum would be helpful along with what you need out of it. From What I understand you are looking to get the title in to the grouping criteria and that should be doing by having a compound grouping criteria i.e.
_id: {category: "$category", title: "$title"}
If the title is within an array, you should do unwind, group and then wind again to achieve the result.