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Steps To Reproduce:

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Here's the following example.

unwind
{ path: '$data', includeArrayIndex: 'index', preserveNullAndEmptyArrays: true }
sort
{ "_id": 1 }
where _id are sequence of numbers 1,2,3,4 etc

group

{
_id:"$id", //$id is objectid
title:
{ $first: '$title' }
,
description:
{ $first: '$description' }
,
episode:
{ $first:'$episode' }
,
seasons:
{ $first: '$seasons' }
,
episodes:
{ $first:"$episodes" }
,
dur:
{ $first: '$dur' }
,
"4k":
{ $first: '$4k' }
,
hlghdr:
{ $first: '$hlghdr' }
,
progress:{$first:null},
image:
{ $first:"$image" }
,
watchlist:
{ $first:"$watchlist" }
,
type:
{ $first: '$type' }
,

data:
{ $push: '$data' /// <- here the sequence gets changed when using addToSet }
}

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Here's the following example. unwind { path: '$data', includeArrayIndex: 'index', preserveNullAndEmptyArrays: true } sort { "_id": 1 } where _id are sequence of numbers 1,2,3,4 etc group { _id:"$id", //$id is objectid title: { $first: '$title' } , description: { $first: '$description' } , episode: { $first:'$episode' } , seasons: { $first: '$seasons' } , episodes: { $first:"$episodes" } , dur: { $first: '$dur' } , "4k": { $first: '$4k' } , hlghdr: { $first: '$hlghdr' } , progress:{$first:null}, image: { $first:"$image" } , watchlist: { $first:"$watchlist" } , type: { $first: '$type' } , data: { $push: '$data' /// <- here the sequence gets changed when using addToSet } }

While using group function of aggregation, the $addToSet for an array randomizes the sequence.

Where as $push keeps the sequence as it is.