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MongoDB $ where queries and tail cursors are WAS: best practice regarding date

My problem: give me a list of documents older than X time.

If I have a document created with

db.dates.insert({date: new Date()});

And now I want to find it only when the “date” has become 30 minutes:

db.dates.find({ $where: "this.date.getTime() + 30 * 60000 <= new Date()"});

This works, but the Mongo docs say quite clearly that there is a significant performance hit for $ where the requests are.

So the question is, is there a better way?

========== UPDATE 1 ===========

I should add that I hope that this query function “dynamically” will create a query once and use it to get the tail cursor on a closed collection ... and I'm not sure that this is actually possible.

I will test and rewrite.

========== UPDATE 2 ===========

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if (cursor.hasNext()) {
    DBObject obj = cursor.next();

    if (config.getQueueDelay() > 0) {
        ObjectId objId = (ObjectId) obj.get("_id");
        long advisedSleep = (objId.getTime() + config.getQueueDelay() * 60000)
        - System.currentTimeMillis();
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var threshold = new Date(); 
threshold.setMinutes(-30);

// now, query against a constant:
db.dates.find({"date" : {$lte : threshold}});

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var c = db.Collection.Find(Query.LTE("date", DateTime.UtcNow.AddMinutes(-30));
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, Mongo, , $where javascript .

I suppose this really makes sense, given the fact that you can reference the "this" object - if javascript has not been reinstalled each time, then "this" actually means "first."

The effectiveness of this remains to be seen, especially since more and more tail-tails are being added to the collection.

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