Mongodb $lookup Not working with _id

Question

wend try with this query, return the lookup is empty

db.getCollection(\'tests\').aggregate([
    {$match: {typet:\'Req\'}},
    {$project: {incharge:1}},
            


        

Answer 1

First, assert that the type of the incharge field is mongoose.Schema.Types.ObjectId. If you still get an empty array back it might be because you are using the schema name you declared in NodeJS instead of the collection name used by MongoDB.

Example from a UserSchema file:

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const UserSchema = new Schema({
  name: { 
    type: String, 
    required: true
  },
  incharge: { 
    type: Schema.Types.ObjectId, 
    required: true
  },
})

const User = mongoose.model('User', UserSchema)
module.exports = User

The model above is named User by mongoose but the corresponding collection in mongoDB is named users. The resulting $lookup is written as:

$lookup:{
  from: "users",           // name of mongoDB collection, NOT mongoose model
  localField: "incharge",  // referenced users _id in the tests collection
  foreignField: "_id",     // _id from users
  as: "user"               // output array in returned object
}

https://mongoosejs.com/docs/models.html
https://mongoosejs.com/docs/schematypes.html