How to project array index after unwinding an array with MongoDB aggregation framework

Question

Is it possible to access the array index when unwinding an array (http://docs.mongodb.org/manual/reference/operator/aggregation/unwind/#pipe._S_unwind) using the MongoDB aggregation pipeline?

For example, suppose I'm unwinding this document in collection "c":

{_id: 1, elements: ["a", "b", "c"]}

then this operation:

db.c.aggregate([
 {$unwind: "$elements"}
])

will return a cursor for documents:

[
 {_id: 1, elements: "a"},
 {_id: 1, elements: "b"},
 {_id: 1, elements: "c"}
]

I'd like to be able to figure out aftewards that "a" had index 0, "b" had index 1 and "c" had index "2" in the original array before unwinding.

How can I project the array index in an unwinding operation?

Alain1405 · Accepted Answer · 2016-01-08 11:58:17Z

15

The newly released MongoDB 3.2 supports unwinding of the array index.

Instead of passing a path the $unwind operator, you can pass an object with the field path and the field includeArrayIndex which will hold the array index.

From MongoDB official documentation:

{
  $unwind:
  {
    path: <field path>,
    includeArrayIndex: <string>,
    preserveNullAndEmptyArrays: <boolean>
  }
}

answered Jan 8, 2016 at 11:58

Alain1405

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2 Comments

Tokenyet Over a year ago

Is there a possible to reverse this operation in Aggregation? or The solution for projecting array is not possible in Mongodb, but code as Accepted. Sorry, I mean reversing unwind.

Maxím G. Over a year ago

@Tokenyet, shortly: yes, it's possible, e.g. if you unwinded field elements, then you can combine it back like this { $group: { _id: '$_id', elements: { $push: '$elements' } } }. Would be great if you can submit it as a separate question (not just a comment) so that I can give the full answer there.

BatScream · Accepted Answer · 2014-12-27 02:46:59Z

3

Currently this seems to be not possible using the aggregation framework. There is an unresolved open issue linked to it: https://jira.mongodb.org/browse/SERVER-4588.

As a workaround you could use Map-reduce, with the map function assigning an index to each array element.

var map = function(){
    for(var i=0;i<this.elements.length;i++){
    emit({"_id":this._id,"index":i},{"index":i,"value":this.elements[i]});
    }
}

edited Dec 27, 2014 at 2:46

answered Dec 27, 2014 at 2:34

BatScream

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3 Comments

Roland Bouman Over a year ago

Thank you! I got the impression it wasn't possible since I saw now examples nor found any operator to project the index. But I didnt find this issue report yet. Thanks!

BatScream Over a year ago

You're most welcome. A combination of operators, $let and $map can take us close to project manipulated values but due to their scope restrictions, incremental indexing can't be achieved.

Roland Bouman Over a year ago

I'm aware that with map/reduce one can do virtually anything within the limits of the map/reduce implementation. One thing that keeps puzzling me is how to ensure the map result doesn't get too large. With regard to the problem at hand, I think that JIRA case proposes a reasonable way to project the index. So far, I'm experiencing a pretty steep learning curve learning to query MongoDB.

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