How to convert array of objects into nested object in MongoDB aggregation pipeline

Question

I am currently doing a project where my Database Management System is MongoDB. I am writing an aggregation pipeline where there are several stages. I am currently struggling at a particular stage where I want to obtain the following output. MongoDB has so many operator expressions that I am confused about which one to use to achieve this. I have a collection called Styles and Skus which are as follows:

// Schema for styles
const styles = mongoose.Schema({
  id: Number,
  productId: Number,
  name: String,
  sale_price: String,
  original_price: String,
  default_price: Boolean,
}, {
  strict: false,
});

// Schema for skus
const skus = mongoose.Schema({
  id: Number,
  styleId: Number,
  size: String,
  quantity: String,
}, {
  strict: false,
});

Each style can have several SKUs, one-to-many relationships. In my aggregation pipeline, I am using $lookup to find all the SKUs of that particular style and adding a new field called SKUs in the styles document. I am getting results like this after the $lookup.

{
            "style_id": 1,
            "name": "Forest Green & Black",
            "original_price": "140",
            "sale_price": "0",
            "default?": true,
             "skus": [
                { 
                   "id": 37,
                   "styleId": 1,
                   "size": "XS",
                   "quantity": 16 
                },
                { 
                   "id": 38,
                   "styleId": 1,
                   "size": "S",
                   "quantity": 8
                } 
              ]  

}

Which is expected as $lookup returns a matching array. But I want my Styles document to look like this.

{
            "style_id": 1,
            "name": "Forest Green & Black",
            "original_price": "140",
            "sale_price": "0",
            "default?": true,
             "skus": {
                  "37": {
                    "styleId": 1,
                   "size": "XS",
                   "quantity": 16 
                  },
                  "38": {
                   "styleId": 1,
                   "size": "S",
                   "quantity": 8
                  }
               }
                    

}

Can someone give any idea how to structure the data like above in aggregation pipeline? Any help would be greatly appreciated. Thanks in advance.

Answer 1

You could use $map or $reduce to convert the objects in the array to the form

[
 {
  k: <key1>
  v: <value1>
 },
 {
  k: <key2>
  v: <value2>
 }
]

Then use $arrayToObject which will convert the array to an object like:

{ 
    <key1>: <value1>,
    <key2>: <value2> 
}

Together, this might look like:

{$addFields: {
    skus: {
      $arrayToObject: {
        $map: {
          input: "$skus",
          in: {
            k: {$toString: "$$this.id"},
            v: "$$this"
          }
        }
      }
    }
}}

Playground