Can a subdocument be required in mongoose?

Yes, your schema is correct.

The docs for mongoose nested schema (SubDocuments) can be found here

i suppose you'll update eventSchema with subdocuments of type user model. you can use { runValidators: true} for update.

eventModel.update({ name: 'YOUR NAME' }, { $push: { host: user } }, { runValidators: true}, function(err) {

})