MongoDB nested collection query of same collection - mongodb

Question

How do you do a nested query when you need the results of the same collection in the next query?
var mongo = require('../config/mongo');
var mongoDB = mongo.db;
...
exports.myFunction = function(req, res) {
...
...
// e.g. myArray = ['a','b','c'];
mongoDB.collection('MyCollection', function(err, collection) {
collection.find({ $or: [{ 'source': {$in: myArray} },{ 'target': {$in: myArray} }]}, { "someVar": 0}).toArray(function(err, firstResults) {
var allResults = [];
for (var i = 0; i < firstResults.length; i++) {
allResults[firstResults[i].source]=1;
allResults[firstResults[i].target]=1;
};
var secondResults = Object.keys(allResults);
mongoDB.collection('MyCollection', function(err, collection) {
collection.find({ $or: [{ 'source': {$in: secondResults} },{ 'target': {$in: secondResults} }]}, { "someVar": 0}).toArray(function(err, items) {
res.send(items);
});
});
});
});
But it doesn't like that I am calling the same collection 'MyCollection' twice. I'm trying to get any document whose source or target involves secondResults.

Answer 1

You should get rid of the second call to open 'MyCollection'. You already have a 'MyCollection' object after opening it once. Namely, you have this:
mongoDB.collection('MyCollection', function(err, collection) {
collection.find({ $or: [{ 'source': {$in: secondResults} },{ 'target': {$in: secondResults} }]}, { "someVar": 0}).toArray(function(err, items) {
res.send(items);
});
});
but you already have an instance of 'MyCollection' from here:
mongoDB.collection('MyCollection', function(err, collection) {
collection.find({ $or: [{ 'source': {$in: myArray} },{ 'target': {$in: myArray} }]}, { "someVar": 0}).toArray(function(err, firstResults) {
So since the second call to 'collection' is in the scope of the first call, you can (and should) reuse that instance:
collection.find({ $or: [{ 'source': {$in: secondResults} },{ 'target': {$in: secondResults} }]}, { "someVar": 0}).toArray(function(err, items) {
res.send(items);
});
Alternatively, if you're using node-mongodb-native, you can have a look at some of the examples to see other ways of accomplishing what you're after.
UPDATE 1
This code works for me:
var MongoClient = require('mongodb').MongoClient;
MongoClient.connect('mongodb://127.0.0.1:27017/test', function(err, db){
if(err) throw err;
var collection = db.collection('c');
var secondQuery = {};
collection.find({a:'b'}).toArray(function(err, firstRes){
for(var i = 0; i < firstRes.length; i++){
secondQuery[firstRes[i].a] = firstRes[i].b;
}
collection.find(secondQuery).toArray(function(err, secondRes){
for(var i = 0; i < secondRes.length; i++){
console.log(secondRes[i]);
}
db.close();
});
});
});
UPDATE 2
Note that the above code is using node-mongodb-native version 1.3.19. Since version 1.2, it is recommended to use MongoClient when interacting with mongodb from Node.js driver.