1// virtual pupulate
2AuthorSchema.virtual('posts', {
3  ref: 'BlogPost',
4  localField: '_id',
5  foreignField: 'author'
6})
7
8// add this in your schema if vritual is not work, this method working for me
9AuthorSchema.set('toObject', { virtuals: true })
10AuthorSchema.set('toJSON', { virtuals: true })