MongoDB Aggregation - Does $unwind order documents the same way as the nested array order

Question

I am wandering whether using $unwind operator in aggregation pipeline for document with nested array will return the deconstructed documents in the same order as the order of the items in the array. Example: Suppose I have the following documents

{ "_id" : 1, "item" : "foo", values: [ "foo", "foo2", "foo3"] }
{ "_id" : 2, "item" : "bar", values: [ "bar", "bar2", "bar3"] }
{ "_id" : 3, "item" : "baz", values: [ "baz", "baz2", "baz3"] }

I would like to use paging for all values in all documents in my application code. So, my idea is to use mongo aggregation framework to:

1. sort the documents by _id
2. use $unwind on values attribute to deconstruct the documents
3. use $skip and $limit to simulate paging

So the question using the example described above is:

Is it guaranteed that the following aggregation pipeline:

[
    {$sort: {"_id": 1}},
    {$unwind: "$values"}
]

will always result to the following documents with exactly the same order?:

{ "_id" : 1, "item" : "foo", values: "foo" }
{ "_id" : 1, "item" : "foo", values: "foo2" }
{ "_id" : 1, "item" : "foo", values: "foo3" }
{ "_id" : 2, "item" : "bar", values: "bar" }
{ "_id" : 2, "item" : "bar", values: "bar2" }
{ "_id" : 2, "item" : "bar", values: "bar3" }
{ "_id" : 3, "item" : "baz", values: "baz" }
{ "_id" : 3, "item" : "baz", values: "baz2" }
{ "_id" : 3, "item" : "baz", values: "baz3" }

Answer 1

I also asked the same question in the MongoDB community forum . An answer that confirms my assumption was posted from a member of MongoDB stuff.

Briefly:

Yes, the order of the returned documents in the example above will always be the same. It follows the order from the array field.

Answer 2

In the case that you do run into issues with order. You could use includeArrayIndex to guarantee order.

[
 {$unwind: {
   path: 'values',
   includeArrayIndex: 'arrayIndex'
 }},
 {$sort: {
   _id: 1,
   arrayIndex: 1
 }},
 { $project: {
    index: 0
 }}
]