[{"_id":0,"id":3423423,"lotsOfFields":"with stuff","voters":[{"idCard":123,"otherFields":"other stuff"},{"idCard":234,"otherFields":"other stuff"},{"idCard":345,"otherFields":"other stuff"},{"idCard":456,"otherFields":"other stuff"},{"idCard":567,"otherFields":"other stuff"},]}]

db.collection.find({"id":3423423,"voters.idCard":345},{"voters.$":1})