MongoDB - Read array objects and return only the matching objects

Question

I want to write a mongodb query that allows me to fetch array objects fields based on the IN/OR condition like in relational databases. If I have the following document in my collection, I want to read all "events.event" in ('user','bot')

{_id: 1,
sender_id:100,
"events" : [
            {
                    "event" : "action",
                    "timestamp" : 1619463803.7244627,
                    "name" : "abc1"
            },
 {
                    "event" : "user",
                    "timestamp" : 1619463803.7244627,
                    "name" : "abc2"
            },
 {
                    "event" : "bot",
                    "timestamp" : 1619463803.7244627,
                    "name" : "abc3"
            }

}

I used the following query but it only works for ONE event at a time. Can this be modified to consider event = 'user' or event = 'bot' ? And can we also project events.event and events.timestamp along with this elemMatch; all in one go?

db.conversations.find(
{"events.event": "bot"}, 
{_id: 0, sender_id:1, events: {$elemMatch: {event: "bot"}}});

no, that returns all the array objects and not just the one with event= bot — Priyanka S
– Priyanka S, Commented Apr 29, 2021 at 8:59
You can use the $filter aggregation expression operator in your projection. — prasad_
– prasad_, Commented Apr 29, 2021 at 9:27

Robot · Accepted Answer · 2021-04-29 09:34:54Z

2

you have lots of options

the simplest one that simulates what you have in mind about SQL queries with IN operator

db.putCollectionNameHere.aggregate([
  { $match: { _id: 1 } },
  { $unwind: '$events' },
  { $match: { 'events.event': { $in: ['user', 'bot'] } } },
]);

about $unwind documentation says

$unwind

Deconstructs an array field from the input documents to output a document for each element. Each output document is the input document with the value of the array field replaced by the element.

answered Apr 29, 2021 at 9:34

Robot

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1 Comment

Priyanka S Over a year ago

Thank you! I tried the above query step-by-step on MongoDB Compass aggregation pipeline and it worked.

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