aggregate with $match and $lookup in nodeJS and mongoDB

Question

I am hitting http://localhost:4000/api/getsinglepost/6199ae17a515fca1c132c901

This code give me empty array [] .

When i removed $match give me all posts with "comment_section": []

I am uploading post and comment table screenshots as well as the response i need .

exports.singlePost = async (req, res) => {

      const getit = await postTable.aggregate([
        {
          $match: { _id: req.params.id }
        },
        {
          $lookup:
           {
             from: "commenttables",
             localField: "_id",
             foreignField: "postID",
             as: "comment_section"
           }
        }
      ])
      .then(data => {
        res.send(data);
      }).catch(err => {
        res.status(500).send({
          message: err.message || "something wrong while getting details"
        });
      });
};

My post table:-

My comment table:-

Response i need like:-

Answer 1

MongoDB query operators are type-sensitive. req.param.id most likely contains a string, while the _id fields contain ObjectId , so they don't match.

You can convert the string to ObjectId using the bson module that comes with the mongodb node.js driver.

After requiring the driver

const mongo = require("mongodb")

Convert with:

mongo.ObjectId(req.param.id)