How to save aggregation change?

Question

I am executing the following:

db.emails.aggregate(
 [
    {$addFields : {arr : {$objectToArray : "$$ROOT"}}}, 
    {$project : { pass : {$slice : ["$arr.v", 1, 20 ] }}}
]
).pretty()

When I exit the session / shell, the changes are not saved.

Can someone please direct me on how to apply this modification to the entire collection and save it?

Use [$out](https://docs.mongodb.com/manual/reference/operator/aggregation/out/) — s7vr, Jan 24 '18 at 01:20
Possible duplicate of [Aggregate and update MongoDB](https://stackoverflow.com/questions/35229566/aggregate-and-update-mongodb) — s7vr, Jan 24 '18 at 01:22

score 0 · Answer 1 · answered Jan 24 '18 at 04:17

looks like continuation of How to merge multiple fields in a collection?

you can $out to new collection,check data,rename to old collection name dropping old data

db.emails.aggregate(
 [
    {$addFields : {arr : {$objectToArray : "$$ROOT"}}}, 
    {$project : { pass : {$slice : ["$arr.v", 1, 20 ] }}},
    {$out : "new_emails"} // out to new collection
]
).pretty()

db.new_emails.renameCollection('emails', true); // rename collection, true - dropTarget

How to save aggregation change?

1 Answers1