Count recurrences in a collection and merge aggregation result to another collection - mongodb

Question

I need to count the recurrences of a value in the collection A, so I do
db.collectionA.aggregate( [ { $group : { name : "$name", count :{$sum: 1 } } } ] )
And have something like
{
"name": "Bruce",
"count": 2
},
{
"_id": "Alfred",
"count": 3
}
Then I need to get this result and populate a field of the collection B, I imagine something like a forEach but don't know how to implement
db.collectionB.findAndModify({query: {"name":forEach of the pervious result},
update:{"nameRecurrences": value of the count}})

Answer 1

Looking at your aggregation pipeline :
db.collectionA.aggregate( [
{ $group : { _id : '$name', name : {$first : "$name"}, nameRecurrences :{$sum: 1 } } }, // renamed field `nameRecurrences` to match with field name in `collection-B`
{$project : {_id : 0}} ] ) // Removing _id to avoid conflicts on merge
On MongoDB version >= 4.2 you can use $merge aggregation operator to merge result of aggregation pipeline on one collection to another collection :
Just add below stage as last stage of aggregation pipeline :
{$merge : { into: { db: "dbName", coll: "collectionB" }, on: "name", whenNotMatched: "discard"}} // Remember to create unique index on `name` field on `collectionB`
Since you're using MongoDB version 3.6.16 :
If the collection has to be created now then you can use $out, but if it's an existing collection with lot of fields in each document apart from just name & nameRecurrences then you can try this in code :
Since you've different filters and their respective update part, then you can take advantage of .bulkWrite() to update multiple documents :
let bulkArr = []
for (const i of aggregationResult){
bulkArr.push( { updateOne : {
"filter" : { "name" : i.name },
"update" : { $set : { "nameRecurrences" : i.nameRecurrences } }
} })
}
db.collectionB.bulkWrite(bulkArr)