Joining two collections in mongodb

Your $lookup query of users is overwriting the comments array. Its not working as you think it'll.

You need to unwind the comments array and then run that $lookup of users and then group by the products.

Edit: I have updated the query with $group by code too. Also you can playa around with the query here:https://mongoplayground.net/p/2EA-Glz8Hrm

Product.aggregate([  {    $match: {      _id: "5cc9441feed4c258881c99cd"    }  },  {    $lookup: {      from: "comments",      localField: "_id",      foreignField: "product",      as: "comments"    }  },  {    $unwind: "$comments"  },  {    $lookup: {      from: "users",      localField: "comments.user",      foreignField: "_id",      as: "comments.user"    }  },  {    $unwind: "$comments.user"  },  {    $group: {      _id: "$_id",      // add other fields you want to include      comments: {        $addToSet: "$comments"      }    }  },])

As suggested by Hamza, I made following changes to my query

Product.aggregate([      {        $match: {          _id: mongoose.Types.ObjectId(id)        }      },      {        $lookup: {          from: "comments",          localField: "_id", //field from input document          foreignField: "product", // field from documents of the 'from' collection          as: "comments"        }      },      {        $unwind: "$comments"      },      {        $lookup: {          from: "users",          localField: "comments.user", //field from input document          foreignField: "_id", // field from documents of the 'from' collection          as: "comments.user"        }      },      {        $unwind: "$comments.user"      },      {        $group: {          _id: "$_id",          title: { $first: "$title" }, // $first returns the first expression of the document it encounters, ex. first title          price: { $first: "$price" },          imageUrl: { $first: "$imageUrl" },          description: { $first: "$description" },          rating: { $first: "$rating" },          comments: {            $addToSet: "$comments" // group comments and create an array          }        }      },      {        $project: {          _id: 1,          title: 1,          price: 1,          imageUrl: 1,          description: 1,          rating: 1,          comments: {            _id: 1,            text: 1,            date: 1,            user: {              _id: 1,              name: 1            }          }        }      }    ])

With this I got the desired result.