how to get data from 3 collections in mongodb using node js?

Question

I have the below collections in mongodb

db.orders.find()

{ \"_id\" : ObjectId(\"5cc69ad493297eade15bacb7\"), \"item\" : \"card\         


        

Answer 1

There are 2 issues in your query:

1. The field user_mob (orders schema) is a Number while mobile_no (user_nippon schema) is a String therefore the lookup cannot be made. You might want to use the same type for those fields.

2. The second lookup is incorrect. the first lookup and unwind returns your item element inside the orders_data element so the localField of this lookup should be changed to: orders_data.item

So after you change both user_mob and mobile_no to have matching types your query would look like this:

db.collection('users_nippon').aggregate([
            {
                $lookup: {
                   from: "orders",
                   localField: "mobile_no",
                   foreignField: "user_mob",
                   as: "orders_data"
                }
            },
            {
                $unwind: "$orders_data"
            },
            {
                $lookup: {
                    from: "items",
                    localField: "orders_data.item",
                    foreignField: "item",
                    as: "items_data"
                }
            },
            {
                $unwind: "$items_data"
            }
        ]).toArray(function(err, list) {
            if (err) throw err;
            console.log(JSON.stringify(list));
            res.send(JSON.stringify(list));
            });