Mongoose Populate values and modifying the Object

Question

I am trying to populate the find results using mongoose. But I am facing an issue.

My data is a list of products and each and every product contains a list of variants. Suppose I have a product named Phone and I have a list of variants named blue, black and green. What I want to do is to return a complete list of products and a sum of stocks.

An Example of product Object is:

{
 "name" : "Phone",
 "details" : "blaah blaah",
 "variants" : [
               ObjectId("5e22d3dabb6cc53debc3a4ae"), 
               ObjectId("5e23629c82aac939fe95c926") 
              ]
}

And an example of variant Object is

 [
  {
   "name" : "blue",
   "stocks": 14,
   "cost" : 200
  },
  {
   "name" : "black",
   "stocks": 9,
   "cost" : 202
  }
 ]

Currently I am doing the following

router.get("/fetchProducts", (req, res) => {
    Product.find({}).populate("variants")
        .then(products => {
            res.send(products);
        }).catch(err => {
            res.send(err)
        })
})

But I want to add the stocks of all the variants and append it to the result. Like this

{
 "name" : "Phone",
 "details" : "blaah blaah",
 "variants" : [
               {
                "name" : "blue",
                "stocks": 14,
                "cost" : 200
               },
               {
                "name" : "black",
                "stocks": 9,
                "cost" : 202
                } 
              ],
 "stocks": 23
}

Answer 1

You can use an aggregation pipeline with a $reduce operator but before that you have to populate the documents, you can do that with $lookup which is similar to populate.Here is an example with your data. I haven't tested but I hope it works.

Product.aggregate([{
     $lookup: {
        from: "variants",
        localField: "variant",
        foreignField: "_id",
        as:"variant"
      }

}, {
        "$addFields": {
            "stocks": {
                "$reduce": {
                    "input": "$variants",
                    "initialValue": 0,
                    "in": { "$add" : ["$$value", "$$this.cost"] }
                }
            }
        }
    }
]).exec(function(err, data) {
    if(err) console.log(err)
    console.log(data)

})