[{"_id":1,"key":["1-value-1","1-value-2"]},{"_id":2,"key":["2-value-1","2-value-2"]}]

/** add a new element to key in _id: 1.* eg: update to 1-value-3 if 1-value-2 exists* else push 1-value-3 if 1-value-4 doesn't exist*/db.collection.find()