Mongo export query to project only specific field in the json object within json array

Question

I have a requirement to project only specific field in the json object within json array. Let's say I have a collection with many documents like below

{
 "_id" : Object("sddf3r"),
 "item_id" : "1235",
 "outerObj" : [{
    "fieldA" : "valueA",
    "fieldB" : "valueB",
    "created_at" : "2019-07-10T14:25:30.000Z"  
 }]
}

Now I want to export the fields item_id,outerObj.created_at as csv and i use the below query for it

mongoexport --host="localhost:27017" -d testdb -c items --query '{"item_id" : {$in :["1235"]}}' --csv -f item_id,outerObj.created_at --out output.csv

But this results in the whole outerObj get printed.

How should i modify the query to export only the field created_at inside the outerObj ?

Answer 1

If you are happy with exporting first item from outerObj array only you can use index in the field list:

mongoexport \
    --host="localhost:27017" \
    -d testdb \
    -c items \
    --query '{"item_id" : {$in :["1235"]}}' \
    --csv \
    -f item_id,outerObj.0.created_at \
    --out output.csv