Mongoose CastError when using $in with ObjectId array on referenced field

[Suraj_Singh_N_A]

My workflow goes like this i fetch data from my login schema like this

const userSchema = new mongoose.Schema({
username: String,
password: String,
department: { type: Schema.Types.ObjectId, ref: ‘departments’ },
// other fields…
});

const Login = mongoose.model(‘Login’, userSchema);

const employeeSchema = new mongoose.Schema({
login_id: { type: mongoose.Schema.Types.ObjectId, ref: ‘Login’ },
employeeCode: String,
firstName: String,
// other fields…
});

const BasicEmployee = mongoose.model(‘basic-employee’, employeeSchema);

const fetchData = await Login.find({ department: department_id });
const loginIds = fetchData.map(item => item._id);
console.log('loginIds:', loginIds, loginIds.every(id => id instanceof mongoose.Types.ObjectId));

and then it returns data like this

loginIds: [
new ObjectId(‘67fcaee2d7d3b3e80fa8d45b’),
new ObjectId(‘67fcaee4d7d3b3e80fa8d51b’),
new ObjectId(‘67fcaee6d7d3b3e80fa8d59b’),
new ObjectId(‘67fcaeead7d3b3e80fa8d6db’),
new ObjectId(‘67fcaeefd7d3b3e80fa8d8ab’),
new ObjectId(‘67fcaeefd7d3b3e80fa8d8bb’)
] true

now when i try to find from the basic employee schema usign login ids

 const employees = await BasicEmployee.find({
        login_id: { $in: loginIds }
      });

it returns error like this

CastError: Cast to ObjectId failed for value “{
‘$in’: [
new ObjectId(‘67fcaee2d7d3b3e80fa8d45b’),
…
]
}” (type Object) at path “login_id”

upon further debugging i found that login id should be a valid object id in basicemployee schema

i furthter check these things

All _ids in loginIds are valid ObjectIds.

login_id field in basic-employee schema is of type ObjectId and correctly references Login.

No stringified JSON, all ObjectIds are real and direct.

Logging typeof loginIds[0] gives “object” and instanceof mongoose.Types.ObjectId is true.

Am I missing something obvious? Is there a deeper reason this $in is being casted as a string object?

[Suraj_Singh_N_A]

const employees = await BasicEmployee.collection
.find({ login_id: { $in: loginIds } })
.toArray();

i was able to use $in using collection but why it is not working without collection ?