MongoDB $lookup Objectid get empty array?

Question

I\'m new to mongodb and practice aggregate with $lookup in mongoose following document, And i met a question really confusing.

I have user

Answer 1

The from field in $lookup is the collection name, not a model variable name. So if you are initializing the model like this

db.model('User', userSchema)

then the lookup query should be

commentModel.aggregate([{$lookup: {
  from: 'users',
  localField: 'author',
  foreignField: '_id',
  as: 'common'
}])