Mongodb $lookup inside $addfield?

user_16325759 0
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I have a collection named users, and this is how one specific user will look like:

{
    _id: 'Object ID',
    name: 'String',
    cart: [
        {
            product_id: 'Product object ID',
            quantity: 'Number',
        },
        ...
    ],
}

I want my desired results to look like this:

{
    _id: 'Object ID',
    name: 'String',
    cart: [
        {
            product_id: 'Product object ID',
            quantity: 'Number',
            product_details: {
                'all the details of the product from Products collection which matches the product_id',
            },
        },
        ...
    ],
}

I tried adding addFields into lookup but it's getting too complicated and doesn't work as desired. What's the best way to aggregate this?

mongoose node.js mongodb
1 Answers

You can achieve this in several different ways, here's what I consider to be the most simple:

db.users.aggregate([
  {
    "$lookup": {
      "from": "products",
      let: {
        cart: "$cart"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $in: [
                "$_id",
                "$$cart.product_id"
              ]
            }
          }
        },
        {
          $replaceRoot: {
            newRoot: {
              "$mergeObjects": [
                "$$ROOT",
                {
                  "$arrayElemAt": [
                    {
                      $filter: {
                        input: "$$cart",
                        cond: {
                          $eq: [
                            "$_id",
                            "$$this.product_id"
                          ]
                        }
                      }
                    },
                    0
                  ]
                }
              ]
            }
          }
        }
      ],
      "as": "cart"
    }
  }
])

Mongo Playground

user_107703700
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