Why don't populations work with String instead of ObjectID?

Question

MongoDB

Good evening everyone. why don't populations work with mongoose.Schema.Types.String? is it possible at all? code such
model Tweet.js

hashtags: [{ type: mongoose.Schema.Types.String, ref: 'Hashtag' }]

those. I store in the Tweets collection not the hashtag IDs, but the names (they are unique) everything works fine until the moment I do the populations.

"Cast to ObjectId failed for value \"one\" at path \"_id\" for model \"Hashtag\""

those. it still climbs into _id