How do I store the aggregation result into another database - mongodb

Question

I can store an aggregation result into another collection within the same database.
But how can I store the result into another database?
This is for copying a collection into another database:
use test1;
db["user_data"].find().forEach(
function(d){ db.getSiblingDB("test2")['user_data'].insert(d);
});
Aggregation function:
pipeline = [
{
'$group': {
'_id': {
'$year': '$birthday'
},
'count': {
'$sum': 1
}
}
}, {
'$sort': {
'_id': 1
}
}, {
'$out': output_collection
}
];
cur = db[source_collection].runCommand('aggregate', {
pipeline: pipeline,
allowDiskUse: true
});

Answer 1

After running the aggregation to the output collection, you need to run another command that clones the collection to another database using db.cloneCollection() as follows:
db.runCommand({
cloneCollection: "test.output_collection",
from: "mongodb.example.net:27017",
query: { active: true }
})
The above copies the output_collection collection from the test database on the server at mongodb.example.net. The operation only copies documents that satisfy the query { active: true } but the query arguments is optional. cloneCollection always copies indexes.

Answer 2

Starting Mongo 4.2, the new $merge aggregation operator can be used to write the result of an aggregation pipeline to the specified collection within another database:
db.collection.aggregate([
// { $group: { "_id": { $year: "$birthday" }, "count": { $sum: 1 } } },
// { $sort: { "_id": 1 } },
{ $merge: { into: { db: "to", coll: "collection" } } }
])
Note that if the targeted collection already contains records, the $merge operator comes with many options to specify how to merge inserted records conflicting with existing records.