why am I only able to see one side data using aggregate function in mongodb?

Question

Image

As you all can see the in the image the postImage is showing Array(0), I have used the following code.

dbs.collection('posts').aggregate([
        {
            $lookup:{
                from: "images",
                localField: "_id",
                foreignField: "object_id",
                as: "postImage"
            }
        },
    ]).toArray((err, res) => {
        if(err) throw err;
        doc.send(JSON.stringify(res));
        dbs.close();
    })

Image of databse:-

post collection

image collection

Here posts collections _id matches to the object_id with the images collection. I want to fetch both the data. what should I do?

Thank You in advance.

Answer 1

I think so it will work after modifying code to this.

 {
  "$project": {
    "_id": {
        "$toString": "$_id"
       }
    }
 },

before using $lookup