MongoDB apply iterator with additional query to all results - node.js

Question

Is there anyway within mongo, via MapReduce or Aggregation to apply a second query based on the result set of the first?, such as an Aggregate within an aggregate, or new emit/query within MapReduce.
For example, I have a materialized path pattern of items (which also includes parentId), I can get all of the roots simply by:
db.collection.find({parentId: null}
.toArray(function(err, docs) {
});
What I want to do is determine if these docs have children, just a flag true/false. I can iterate through these docs using async each and check, but on large docs, this is not very performant at all and causes event loop delays, I can use eachSeries, but this is just slow.
Ideally, I'd like to be able to handle this all within Mongo. Any suggestions if that's possible?
Edit, Example collection:
{
_id: 1,
parentId: null,
name: 'A Root Node',
path: ''
}
{
_id: 2,
parentId: 1,
name: 'Child Node A',
path: ',1'
}
{
_id: 3,
parentId: 2,
name: 'Child Node B',
path: ',1,2'
}
{
_id: 4,
parentId: null,
name: 'Another Root Node',
path: ''
}
This basically represents two root nodes, where one root node ({_id: 1}) has two children (one being direct), example:
1
2
3
4
What I would like to do is do a query based on parentId so I can get the root nodes by using null or by passing a parentId I can get the children of that and determine if the result set from this, any of the items contain children, example response for where {parentId: null}:
[{
_id: 1,
parentId: null,
name: 'A Root Node',
path '',
hasChildren: true
},
{
_id: 4,
parentId: null,
name: 'Another Root Node',
path '',
hasChildren: false
}]

Answer 1

You could try creating an array of the parentIds from the materialized paths that you can then use in the aggregation pipeline to project the extra field/flag hasChildren.
This can be done by using the map() method on the cursor returned from the find() method. The following illustrates this:
var arr = db.collection.find({ "parentId": { "$ne": null } })
.map(function (e){ return e.path; })
.join('')
.split(',')
.filter(function (e){ return e; })
.map(function (e){ return parseInt(e); }),
parentsIds = _.uniq(arr); /* using lodash uniq method to return a unique array */
Armed with this array of parentIds, you can then use the aggregation framework in particular the $project pipeline which makes use of the the set operator $setIsSubset which takes two arrays and returns true when the first array is a subset of the second, including when the first array equals the second array, and false otherwise:
db.collection.aggregate([
{
"$match": {
"parentId": null
}
},
{
"$project": {
"parentId": 1,
"name": 1,
"path": 1,
"hasChildren": { "$setIsSubset": [ [ "$_id" ], parentIds ] }
}
}
], function (err, res) { console.log(res); });