How can I populate nested array objects in mongoDB or mongoose?

Question

I have an orders collection where each order has the following shape:

  {
    "_id": "5252875356f64d6d28000001",
    "lineItems": [
      { productId: 'prod_007', quantity: 3 }, 
      { productId: 'prod_003', quantity: 2 }
    ]
    // other fields omitted
  }

I also have a products collection, where each product contains a unique productId field.

How can I populate each lineItem.productId with a matching product from the products collection? Thanks! :)

EDIT: orderSchema and productSchema:

const orderSchema = new Schema({
  checkoutId: {
    type: String,
    required: true,
  },
  customerId: {
    type: String,
    required: true,
  },
  lineItems: {
    type: [itemSubSchema],
    required: true,
  },
});

const itemSubSchema = new Schema(
  {
    productId: {
      type: String,
      required: true,
    },
    quantity: {
      type: Number,
      required: true,
    },
  },
  { _id: false }
);

const productSchema = new Schema({
  productId: {
    type: String,
    required: true,
  },
  name: {
    type: String,
    required: true,
  },
  imageURL: {
    type: String,
    required: true,
  },
  price: {
    type: Number,
    default: 0, 
  },
});

Can you post your schema as well?

AdamExchange
– AdamExchange

2022-11-09 17:12:53 +00:00
Commented Nov 9, 2022 at 17:12 — AdamExchange
– AdamExchange, Commented Nov 9, 2022 at 17:12

J.F. · Accepted Answer · 2022-11-09 17:25:19Z

1

I don't know the exact output you want but I think this is what you are looking for:

The trick here is to use $lookup in an aggregation stage.

First $unwind to deconstruct the array and can merge each id with the other collection.
Then the $lookup itself. This is like a join in SQL. It merges the desired objects with same ids.
Then recreate the population using $mergeObjects to get properties from both collections.
And last re-group objects to get the array again.

db.orders.aggregate([
  {
    "$unwind": "$lineItems"
  },
  {
    "$lookup": {
      "from": "products",
      "localField": "lineItems.productId",
      "foreignField": "_id",
      "as": "result"
    }
  },
  {
    "$set": {
      "lineItems": {
        "$mergeObjects": [
          "$lineItems",
          {
            "$first": "$result"
          }
        ]
      }
    }
  },
  {
    "$group": {
      "_id": "$_id",
      "lineItems": {
        "$push": "$lineItems"
      }
    }
  }
])

Example here

With this query you have the same intial data but "filled" with the values from the other collection.

Edit: You can also avoid one stage, maybe it is clear with the $set stage but this example do the same as it merge the objects in the $group stage while pushing to the array.

edited Nov 9, 2022 at 17:25

answered Nov 9, 2022 at 17:17

J.F.

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2 Comments

xandert.93 Over a year ago

Hi thanks for responding. My first concern is, given that no $match is applied, won't the initial $unwind operation unwind every single lineItems array in every single order? Is that really efficient, especially if I have 1000 orders, for example?

J.F. Over a year ago

Yes, that's true, without $match it will do along all the collection. So if you can use $match is better.

Tim · Accepted Answer · 2022-11-09 18:36:47Z

You can use the Mongoose populate method either when you query your documents or as middleware. However, Mongoose only allows normal population on the _id field.

const itemSubSchema = new Schema({
  product: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'productSchema',
  }
});

const order = await orderSchema.find().populate('lineItems.$*.product');
// special populate syntax necessary for nested documents

Using middleware you would still need to reconfigure your item schema to save the _id from products. But this method would automatically call populate each time you query items:

itemSubSchema.pre('find', function(){
  this.populate('product');
});

You could also declare your item schema within your order schema to reduce one layer of joining data:

const orderSchema = new Schema({
 lineItems: [{
  type: {
   quantity: {type: Number, required: true},
   product: {
    type: mongoose.Schema.Types.ObjectId,
    required: true,
    ref: 'productSchema',   
   }
  },
  required: true,
 }]
});

const orders = orderSchema.find().populate('lineItems');

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