Elasticsearch and subsequent Mongodb queries

Question

I am implementing search functionality using Elasticsearch.

I receive "username" set returned by Elasticsearch after which I need to query a collection in MongoDB for latest comment of each user in the "username" set.

Question: Lets say I receive ~100 usernames everytime I query Elasticsearch what would be the fastest way to query MongoDB to get the latest comment of each user. Is querying MongoDB 100 times in a for loop using .findOne() the only option?

(Note - Because latest comment of a user changes very often, I dont want to store it in Elasticsearch as that will trigger retrieve-change-reindex process for the entire document far too frequently)

Answer 1

This answer assumes following schema for your mongo db stored in comments db.

{
  "_id" : ObjectId("5788b71180036a1613ac0e34"),
  "username": "abc",
  "comment": "Best"
}

assuming usernames is the list of users you get from elasticsearch, you can perform following aggregate:

a =[
    {$match: {"username":{'$in':usernames}}},
    {$sort:{_id:-1}},
    {
       $group:
         {
           _id: "$username",
           latestcomment: { $first: "$comment" }
         }
     }
]
db.comments.aggregate(a)

Answer 2

You can try this..

db.foo.find().sort({_id:1}).limit(100);

The 1 will sort ascending (old to new) and -1 will sort descending (new to old.)