Using aggregation query to get list of users with total transaction count and transaction detail as embedded document

Nagesh Jha

I am trying to get a list of users with total transaction count and each user should have latest transaction detail as embedded object using MongoDB's aggregate pipelines to fetch results in GET API.

I have the following database schema:

User: _id, name, phone, address

Product: _id, name, unit_price, description

Transaction: _id, date, product_id(ref to Product), user_id(ref to User), quantity, total_price

Expected Response JSON

[
    {
        name:"",
        phone:"",
        address:"",
        total_transaction:
        latest_transaction_detail: {
            product_id:
            quantity:
            total_price:                                                        
        }
    },
    {
        name:"",
        phone:"",
        address:"",
        total_transaction:
        latest_transaction_detail: {
            product_id:
            quantity:
            total_price:                                                        
        }
    }
]

How do I generate an aggregate query to return the above?

matthPen

You can achieve this by running an aggregation query.

A lookup stage will join your User collection with your Transaction collection (no need to join Product in your expected result). Its pipeline is splitted with $facet, for both get the count result and the latest transaction for that user
A project stage will reshape your data and extract array elements to documents.

Here's such a query :

db.User.aggregate(
[
    {
        $lookup: 
         {
            from: "Transaction",
           let: { userId: "$_id" },
            pipeline: [ 
            {$facet:
              {count:[{$match:{$expr:{$eq:["$$userId","$user_id"]}}}, {$count:"total_transaction"}],
                latest:[
                {$match:{$expr:{$eq:["$$userId","$user_id"]}}},
                {$sort:{date:-1}},
                {$limit:1}]
            } }],
            as: "transactions"
         }
    },
    {
        $project: {
          last_name:1,
          phone:1,
          address:1,
            total_transaction : {
              $let:{
                vars:{
                  count:{
                    $arrayElemAt:["$transactions.count",0]
                    }
                  },
                in:{
                  $arrayElemAt:["$$count.total_transaction",0]
                  }
                }
              },
              latest_transaction : {
              $let:{
                vars:{
                  latest:{
                    $arrayElemAt:["$transactions.latest",0]
                    }
                  },
                in:{
                  $arrayElemAt:["$$latest",0]
                  }
                }


              },    
            }
        },
    ]
);

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