How to lookup subquery documents with another collection moongoose

I need to join user collections two times as same user will post product and purchased user details as sub document.

Product Schema:

{
    product_title: String,
    product_desc: String,
    Product_purchased:[
        {
            userid: mongoose.Schema.ObjectId,
            purchased_date: Date
        }]
    posteduserId: mongoose.Schema.ObjectId
}

Users Schema:

{
    _id: userId,
    name: String,
    Pic: String
}

Example document:

[
    {
        "product_title":"title",
        "product_desc":"desc",
        "Product_purchased":[
            {
                "userid":"5d4hvh7duc7c7c8d9scbe",
                "name":"name",
                "Pic":"url",
                "purchased_date":"Date"
            },
            {
                "userid":"5d4hvh7duc7c7c8d9scbe",
                "name":"name",
                "Pic":"url",
                "puchased_date":"Date"
            }
        ],
        "posteduserId": "5d4hvh7duc7c7c8d9scbe",
        "userid": "5d4hvh7duc7c7c8d9scbe",
        "name": "name",
        "pic": "url",
    }
]

Join same user table with posted userid and subarray of purchased userIds.

Please help me how to crack this one, Thanks in advance.

First you need to fix your Product schema like this to include the ref field:

const mongoose = require("mongoose");

const ProductSchema = new mongoose.Schema({
  product_title: String,
  product_desc: String,
  Product_purchased: [
    {
      userid: {
        type: mongoose.Schema.ObjectId,
        ref: "User"
      },
      purchased_date: Date
    }
  ],
  posteduserId: {
    type: mongoose.Schema.ObjectId,
    ref: "User"
  }
});

module.exports = mongoose.model("Product", ProductSchema);

I setup user model like this:

const mongoose = require("mongoose");

const UserSchema = new mongoose.Schema({
  name: String,
  Pic: String
});

module.exports = mongoose.model("User", UserSchema);

The ref User value must match the model name User in the user model.

Then you need to populate two times like this:

router.get("/products", async (req, res) => {
  const result = await Product.find({})
    .populate("Product_purchased.userid")
    .populate("posteduserId");
  res.send(result);
});

Let's say you have this 3 users:

{
    "_id": "5e133deb71e32b6a68478ab4",
    "name": "user1 name",
    "Pic": "user1 pic",
    "__v": 0
},
{
    "_id": "5e133df871e32b6a68478ab5",
    "name": "user2 name",
    "Pic": "user2 pic",
    "__v": 0
},
{
    "_id": "5e133e0271e32b6a68478ab6",
    "name": "user3 name",
    "Pic": "user3 pic",
    "__v": 0
}

And this product:

{
    "_id": "5e133e9271e32b6a68478ab8",
    "product_title": "product1 title",
    "product_desc": "product1 description",
    "Product_purchased": [
        {
            "purchased_date": "2020-01-06T14:02:14.029Z",
            "_id": "5e133e9271e32b6a68478aba",
            "userid": "5e133df871e32b6a68478ab5"
        },
        {
            "purchased_date": "2020-01-06T14:02:14.029Z",
            "_id": "5e133e9271e32b6a68478ab9",
            "userid": "5e133e0271e32b6a68478ab6"
        }
    ],
    "posteduserId": "5e133deb71e32b6a68478ab4",
    "__v": 0
}

The result will be:

[
    {
        "_id": "5e133e9271e32b6a68478ab8",
        "product_title": "product1 title",
        "product_desc": "product1 description",
        "Product_purchased": [
            {
                "purchased_date": "2020-01-06T14:02:14.029Z",
                "_id": "5e133e9271e32b6a68478aba",
                "userid": {
                    "_id": "5e133df871e32b6a68478ab5",
                    "name": "user2 name",
                    "Pic": "user2 pic",
                    "__v": 0
                }
            },
            {
                "purchased_date": "2020-01-06T14:02:14.029Z",
                "_id": "5e133e9271e32b6a68478ab9",
                "userid": {
                    "_id": "5e133e0271e32b6a68478ab6",
                    "name": "user3 name",
                    "Pic": "user3 pic",
                    "__v": 0
                }
            }
        ],
        "posteduserId": {
            "_id": "5e133deb71e32b6a68478ab4",
            "name": "user1 name",
            "Pic": "user1 pic",
            "__v": 0
        },
        "__v": 0
    }
]