MongoDB: Remove all array entries except the last X entries

Question

Node.JS MongoDB Driver is used. The database entries look like that:

{
    _id: ObjectId("224444202654a21928801814"),
    category: "Test", 
    entries: [
        {
            _id: ObjectId("324444202dd4a21328802214"),
            username: "Tester",
        },
        {
            _id: ObjectId("324444232dd4a21328802215"),
            username: "Tester2",
        }
        [many more...]
    ]
}

There are thousands of objects in the entries array. The last array entries are the newest ones. Now all objects should be deleted inside the entries array except the last 50 entries. Is there a possibility to achieve that with one update/remove call? Maybe with the $slice operator?

I am able to get entries which are not in last 50 records. Now finding the way to delete them. — dikesh
– dikesh, Commented Apr 3, 2014 at 14:37

Anand Jayabalan · Accepted Answer · 2014-04-03 14:27:38Z

1

It's not possible to achieve what you want with the $slice operator. I think using the javascript slice operator within cursor forEach() is your best bet. Try this out:

db.foo.find({}).forEach( function(doc) { 
    db.foo.update(
        {_id:doc._id}, 
        {$set:{entries:doc.entries.slice(doc.entries.length - 50)}}
    ) 
})

Note: The above command would run fine in the Mongo Shell.

answered Apr 3, 2014 at 14:27

Anand Jayabalan

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1 Comment

David Over a year ago

Okay, I will use this solution for the moment.

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