How to populate a field (not in embedded document) from other embedded document in mongoose?

Question

When tried by using populate method e.g. db[schemaName].find().populate("user_id") returns result as null.

1.schema: Users (Non embedded doc and need to populate user_id field)

user_id: { type: Schema.Types.ObjectId, ref: Sample.users } //Refers to sample users

2.schema: Sample (Embedded doc)

users : [{ name : String, address : address, _id:id },....],...