Migrate to new document structure in mongo 3.6 - mongodb

Question

I have to migrate data from a structure from
{
"_id": "some-id",
"_class": "org.some.class",
"number": 1015,
"timestamp": {"$date": "2020-09-05T12:08:02.809Z"},
"cost": 0.9200000166893005
}
to
{"_id": {
"productId": "some-id",
"countryCode": "DE"
},
"_class": "org.some.class",
"number": 1015,
"timestamp": {"$date": "2020-09-05T12:08:02.809Z"},
"cost": 0.9200000166893005
}
The change that is in the new document is the _id field is replaced by a complex _id object (productId : String, country : String).
The country field is to be completed for the entire collection with a specific value - DE.
The collection has about 40 million records in the old format and 700k in the new format. I would like to bring these 40 million to this new form. I’m using mongo 3.6, so I’m a bit limited and I’ll probably have to use the aggregate functions to create a completely new collection, and then remove the old one.
I will be grateful for help on how to do it - how the query that will do it should look like and how to keep these migrated 700k documents.
What I have got so far:
db.productDetails.aggregate(
{$match: {_id: {$exists: true}}},
{$addFields: {"_id": {"productId": "$_id", "country": "DE"}},
{$project: {_id: 1, _class: 1, number: 1, timestamp: 1, cost: 1}},
{$out: "productDetailsV2"}
)
but this solution would only work if I didn't have 700k documents in the new form.

Answer 1

Your query is in the right direction. You may want to modify the $match filter a bit to better catch the old type documents.
db.collection.aggregate([
{
$match: {
"_id.country": {
$exists: false
}
}
},
{
$addFields: {
"_id": {
"productId": "$_id",
"country": "DE"
}
}
},
{
$project: {
"_id": 1,
"_class": 1,
"number": 1,
"timestamp": 1,
"cost": 1
}
},
{
$out: "productDetailsV2"
}
])
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