gistfile1.js

var mongodb = require('mongodb');

// This example selects all documents of a collection and checks if they should be displayed.
// 15 documents should be displayed, but I have to select all from the DB because maybe
// myCheckFunction returns true only for the last 15 elements selected (if any).

// So at some point (if I have seen enough documents) the DB can stop sending me documents,
// i.e. not fetch another batch of 30 documents in this case.

var db = new mongodb.Db('mydb', new mongodb.Server('localhost', 27017, {}));
db.open(function(err, db) {
  db.collection('mycollection', function(err, collection) {
    var count = 0;
    var stream = collection.find({}).streamRecords({fetchSize: 30});
    stream.on('end', function() {
      db.close();
    });
    stream.on('data', function(data) {
      // "myCheckFunction" checks if this document should be displayed or not
      // this may be something I cannot do in MongoDB.
      if (myCheckFunction(data)) { 
        count++;
        doDisplay(data); // ...
        if (count > 15) { // seen enough
          stream.destroy(); // not implemented!
        }
      }
    });
  });
});