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?

Answer 1

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>
  }
}

Answer 2

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]});
    }
}