Mongoose nested populate virtual return null - mongodb

Question

Why does virtual in a nested populate return null?
I have a Post schema and a User schema like this:
Post Schema:
const PostSchema = new mongoose.Schema({
_author_id: {
type: mongoose.Schema.Types.ObjectId,
ref: 'User',
required: true
},
privacy: {
type: String,
default: 'public'
},
...
}, { timestamps: true, toJSON: { virtuals: true }, toObject: { getters: true, virtuals: true } });
PostSchema.virtual('author', {
ref: 'User',
localField: '_author_id',
foreignField: '_id',
justOne: true
});
module.exports = mongoose.model('Post', PostSchema);
User Schema
const UserSchema = new mongoose.Schema({
email: {
type: String,
required: true,
minlength: 12,
unique: true,
validate: {
validator:
(email) => {
const regex = /^\w+([.-]?\w+)*#\w+([.-]?\w+)*(\.\w{2,3})+$/;
return regex.test(email);
},
message: '{VALUE} is invalid.'
}
},
password: {
type: String,
required: true
},
username: {
type: String,
unique: true,
required: true
}
posts: [{
type: mongoose.Schema.Types.ObjectId,
ref: 'Post'
}],
...
}, { timestamps: true, toJSON: { virtuals: true }, toObject: { getters: true, virtuals: true } });
module.exports = mongoose.model('User', UserSchema);
And when I fetch the user to DB:
const user = await User
.findOne({ username })
.populate({
path: 'posts',
model: 'Post',
select: 'description name photos comments createdAt',
options: {
limit: 3,
sort: { createdAt: -1 },
},
populate: {
path: 'author', //virtual <------- returns null but '_author_id' is fine
model: 'User',
select: 'username fullname profilePicture'
}
})
.sort('-createdAt');
Sample returned document
{
...
"posts": [
{
"photos": [],
"comments": [
"5fe96ec48564ce31dcebe669",
"5fe97c43f4169834a48b3851",
"5fe97c726ccf4633006fbeaa"
],
"description": "Gago ka ba hahahaha",
"_id": "5fe96d84178485086090faa9",
"createdAt": "2020-12-28T05:30:44.157Z",
"author": null, // <-----
"id": "5fe96d84178485086090faa9"
}
]
}
Did I miss something? My author virtual works fine in a non-nested populate.

Answer 1

I think you need to add author in the outer populate select.
const user = await User.findOne({ username })
.populate({
path: 'posts',
select: 'description name photos comments createdAt author', // <------- try adding author here
options: {
limit: 3,
sort: { createdAt: -1 },
},
populate: {
path: 'author',
select: 'username fullname profilePicture',
},
})
.sort('-createdAt');
Also, you shouldn't have to add the model in the populate since it already knows which model from the schema.