Create unique ObjectId during $project pipeline - mongodb

Question

I have an aggregation framework query that is summarizing certain document data into a lookup set. Unfortunately, I can't provide the data since it's company-related. Here is the query and data fragment from the last stages of the pipeline:
...
{ $group: { _id: "$SectionId", "Questions": { $addToSet: "$Questions" } } },
{ $unwind: "$Questions" },
which returns data like this: Note that _id is not unique.
{
"_id" : "Tonometry",
"Questions" : {
"MappingId" : "Exophoria",
"PositiveLabel" : "Positive",
"NegativeLabel" : "Negative"
}
},
{
"_id" : "Tonometry",
"Questions" : {
"MappingId" : "Heterophoria",
"PositiveLabel" : "Positive",
"NegativeLabel" : "Negative"
}
},
The next stage in the pipeline is this:
{
$project: {
"_id": 1,
"id": ObjectId(),
"SectionId": "$_id",
"MappingId": "$Questions.MappingId",
"PositiveLabel": "$Questions.PositiveLabel",
"NegativeLabel": "$Questions.NegativeLabel",
}
},
which produces:
{
"_id" : "Tonometry",
"id" : ObjectId("5d1cf66cf526f23524f865c6"),
"SectionId" : "Tonometry",
"MappingId" : "Exophoria",
"PositiveLabel" : "Positive",
"NegativeLabel" : "Negative"
},
{
"_id" : "Tonometry",
"id" : ObjectId("5d1cf66cf526f23524f865c6"),
"SectionId" : "Tonometry",
"MappingId" : "Heterophoria",
"PositiveLabel" : "Positive",
"NegativeLabel" : "Negative"
},
I tried creating a new field Id that has a unique ObjectId but unfortunately just re-uses the same ObjectId in all the nodes. This is important because when I attempt to use $out, it requires a unique _id.
How do I add a unique ObjectId to each node?

Answer 1

Using $out does not need a unique _id, you can use $replaceRoot together with $mergeObjects prior to $out pipeline, this will merge the Question document into the desired document without an _id field and $out will create the _id field for you in the new collection:
[
....
{ "$group": { "_id": "$SectionId", "Questions": { "$addToSet": "$Questions" } } },
{ "$unwind": "$Questions" },
{ "$replaceRoot": {
"newRoot": {
"$mergeObjects": [
{ "Section": "$_id" },
"$Questions"
]
}
} },
{ "$out": "new-collection" }
]