mongodb get entries where id exists multiple times based on count condition - mongodb

Question

I have a collection 'bookings' with the following example structure:
{
"_id" : ObjectId("1"),
"user" : ObjectId("1"),
"event" : ObjectId("1"),
},
{
"_id" : ObjectId("2"),
"user" : ObjectId("1"),
"event" : ObjectId("1"),
},
{
"_id" : ObjectId("3"),
"user" : ObjectId("2"),
"event" : ObjectId("1"),
},
{
"_id" : ObjectId("4"),
"user" : ObjectId("3"),
"event" : ObjectId("1"),
},
{
"_id" : ObjectId("5"),
"user" : ObjectId("4"),
"event" : ObjectId("2"),
},
{
"_id" : ObjectId("6"),
"user" : ObjectId("1"),
"event" : ObjectId("2"),
}
I cant figure out a query that shows all "event" id's in which the same "user" id appears multiple times. something like this:
{
"event": 1,
"user": 1,
"count": 2
}
Does not have to be this exact output, in other words I just want a way to have a query to get all events for which the same "user" id has more than one entry in this "bookings" collection.
Any suggestions? Thanks!

Answer 1

You just need to do grouping and filtering.
In SQL it would be just as simple as
SELECT count(*) as cc, user, event FROMM t1 GROUP BY user, event HAVING cc > 1
In MongoDB, you can use the aggregation framework to do equivalent stuff.
It does the same in 3 different steps in the pipeline: group, filter, project.
db.mycollection.aggregate( [
{ $group: { _id: { user: "$user", event: "$event", }, count: { $sum: 1 } } },
{ $match: { count: { $gt: 1 } } },
{ $project: { _id: 0,
userId: "$_id. user",
event: "$_id.event",
count: 1
}
}
] )
This documentation can help you to understand deeper: https://www.mongodb.com/docs/manual/reference/sql-aggregation-comparison/