Aggregate Query in MongoDb with Node JS

Question

I am new to Mongodb.i have two collections users and products.

In product collection user id is mapped.product collection has productid,producttype,productname and userid as fields.

i need to write aggregate query

Use aggregate query to get all the products based on “product_type” and populate user (Aggregate query must be used)
Use aggregate query to get all the product based on the user city (Aggregate query must be used)

var MongoClient = require("mongodb").MongoClient;
var url = "mongodb://127.0.0.1:27017/";
MongoClient.connect(url, function(err, db) {
  if (err) throw err;
  var dbo = db.db("demo1");
  dbo
    .collection("products")
    .aggregate([{
        $match: {
          product_type: "$product_type"
        }
      },
      {
        $lookup: {
          from: "users",
          localField: "user_id",
          foreignField: "_id",
          as: "userdetails"
        }
      }
    ])
    .toArray(function(err, res) {
      if (err) throw err;
      console.log(JSON.stringify(res));
      db.close();
    });
});

I have tried this but not sure if this is right.

Please help.

Dĵ ΝιΓΞΗΛψΚ · Accepted Answer · 2020-02-22 05:48:30Z

1

what you have is correct for getting products by type. for getting products by user's city, give this pipeline a try:

db.products.aggregate([
{
  $lookup: {
    from: "users",
    let: { user_id: "$user_id" },
    pipeline: [
      { $match: {
          $expr: {
            $and: [ { $eq: [ "$_id", "$$user_id" ] },
                    { $eq: [ "$city", "new york" ] }]}}}],
    as: "user"
  }
},
{
  $match: {
    $expr: { $gt: [ { $size: "$user" }, 0 ] }
  }
}
])

test: https://mongoplayground.net/p/BebeOCd4wLQ

update: here's another way:

db.products.aggregate([
    {
        $lookup: 
        {
            from: "users",
            localField: "user_id",
            foreignField: "_id",
            as: "user"
        }
    },
    {
        $match: {
            "user.city": "new york"
        }
    },
    {
        $project: {
            user: 0
        }
    }
])

edited Feb 22, 2020 at 5:48

answered Feb 22, 2020 at 5:37

Dĵ ΝιΓΞΗΛψΚ

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