How can I get and populate the id ref to the order? I grab the id of the product and user. Then when I use my get method, I will get all the details of the products and users.

For example, my data will show all information

Product
a. title
b. price
c. productImage

User
a. username
b. studentId

I tried to use populate, but I think I’m doing it wrong.

    export const getOrders = async (req,res) =>{
      try {
      const order = await Order.findOne({productId: '634e91d256326381d5716993'})
                                .populate()
                                .exec()
      res.status(400).json(order)
    } catch (error) {
            res.status(404).json({message: error.message})  
    }
       }

OrderSchema

    const OrderSchema = mongoose.Schema({
        productId: {type:  mongoose.Schema.Types.ObjectId, ref: 'Product', required: true},
        buyerId: {type: mongoose.Schema.Types.ObjectId, required: true, ref: 'User'},
        sellerId: {type: mongoose.Schema.Types.ObjectId, required: true, ref: 'User'}
    
    })
    
    export default mongoose.model('Order', OrderSchema) 

ProductSchema

    const ProductSchema = mongoose.Schema({
     
        user_id: {type: mongoose.Schema.Types.ObjectId, required: true, ref: 'User'},
        title: {type: String},
        description: {type: String},
        categories: {type: Array},
        price: {type: Number},
        productImage: {type: String}
    },
    { timestamps: { createdAt: true } }
     )
    
    export default mongoose.model('Product', ProductSchema)

UserSchema

    const UserSchema = mongoose.Schema({
        username: {type: String, unique: true, required: true},
        password: {type: String,  required: true},
        email: {type: String,  unique: true, required: true},
        studentid: {type: String, unique: true, required: true},
        isAdmin:{type: Boolean, default: false},
        },
       { timestamps: { createdAt: true } }
        )
    
    export default mongoose.model('User', UserSchema)

>Solution :

You have to specify what you want to populate inside the .populate() method:

await Order.findOne({productId: '634e91d256326381d5716993'})
  .populate([
    { path: 'productId', select: 'title price productImage' },
    { path: 'buyerId', select: 'username studentid' }
  ])
  .exec()