Retrieve a specific field Mongoose - node.js

Question

I want retrieve the account array if the query find role "elite"
i try with
db.users.aggregate(
{ $match : { "account.role" : "Elite" } }
);
But i have all object...
{
"_id" : ObjectId("7623902143981943"),
"account" : [
{
"role" : "Elite",
"action" : [
"create",
"read",
"update",
"delete"
],
"extra" : {
account:[1,2,3,4]
}
},
{
"role" : "User",
"action" : [
"create",
"read",
"update",
"delete"
],
"extra" : {
account:[10]
}
}
],
}
Can i retrieve only extra Array ( account:[1,2,3,4] ),if it is a positive result from the query? or I have to parse the received object?
(the schema is very simplified, but I have many roles)

Answer 1

You must use $project and $unwind:
//Order of $unwind and $match matters
db.users.aggregate(
{$unwind: "$account"},
{$match : { "account.role" : "Elite" }},
{$project : { "extra.account" : 1}}
);
explanation
$unwind splits the array into different elements. See the effect of
db.users.aggregate({$unwind: "$account"})
then you match the elements with {"account.role": "Elite"}. See the effect of:
db.users.aggregate(
{$unwind: "$account"},
{$match : { "account.role" : "Elite" }}
);
And then you finally project just the desired fields
db.users.aggregate(
{$unwind: "$account"},
{$match : { "account.role" : "Elite" }},
{$project : { "extra.account" : 1}}
);
//You can also remove the _id filed (included by default with:
db.users.aggregate(
{$unwind: "$account"},
{$match : { "account.role" : "Elite" }},
{$project : { _id: 0, "extra.account" : 1}}
);
OLD ANSWER
You must use projection:
db.users.aggregate(
{$match : { "account.role" : "Elite" }},
{$project : { "extra.account" : 1}}
);
Besides, if you are just matching documents, there's no need to use the aggregation framewrok and you can just use:
// No projection here
db.users.find({"account.role" : "Elite"})
or
// Only returns the _id field + "extra.account" field if exists. By default the _id field is included
db.users.find({"account.role" : "Elite"}, { "extra.account" : 1})
// Only returns the "extra.account" field if exists
db.users.find({"account.role" : "Elite"}, { _id: 0, "extra.account" : 1})
Mongodb documentation can be found here and here