Mongo Facet Aggregation with Sum - mongodb

Question

Trying to figure out something simple in this aggregation. The field "totalArrests" under metadata is coming back 0. It's not able to sum this field from the previous stage for some reason. Please advise.
const agg = await KID.aggregate([
{
$group: {
_id: "$source", // group by this
title: { "$last": "$title"},
comments: { "$last": "$comments"},
body: { "$last": "$body"},
date: { "$last": "$date"},
media: { "$last": "$media"},
source: { "$last": "$source"},
count: { "$sum": 1},
arrestCount: { "$sum": "$arrested"},
rescuedCount: { "$sum": "$rescued"},
}
},
// sorting
{
$sort: {date: sort}
},
// facets for paging
{
$facet: {
metadata: [
{ $count: "total" }, // Returns a count of the number of documents at this stage
{ $addFields: {
page: page,
limit: 30,
totalArrests: {$sum: "$arrestCount"}
}},
],
kids: [ { $skip: (page-1)*30 }, { $limit: 30 } ]
}
},
]);
Here is a sample document in the collection.
[
{
_id: 5e8b922aaf5ccf5ac588398c,
counter: 4,
date: 2017-01-01T17:00:00.000Z,
name: 'Steven Tucker',
arrested: 1,
rescued: 0,
country: 'US',
state: 'NH',
comments: 'Sex trafficking of a minor',
source: 'https://www.justice.gov/opa/pr/new-hampshire-man-indicted-sex-trafficking-minor-connection-interstate-prostitution',
title: 'New ....',
body: 'Steven Tucker, 31, ....',
__v: 0,
media: {
title: 'New Hampshire Man Indicted for Sex ...',
open_graph: [Object],
twitter_card: [Object],
favicon: 'https://www.justice.gov/sites/default/files/favicon.png'
},
id: 5e8b922aaf5ccf5ac588398c,
text: 'New Hampshire Man Indicted',
utcDate: '2017-01-01T12:00'
}
]

Answer 1

$count will only provide you the count for number of documents and escapes all the other things.
So, You have to use one more pipeline in $facet in order to get the documents.
{ $facet: {
metadata: [
{ $group: {
_id: null,
total: { $sum: 1 },
totalArrested: { $sum: "$arrestCount" }
}},
{ $project: {
total: 1,
totalArrested: 1,
page: page,
limit: 30,
hasMore: { $gt: [{ $ceil: { $divide: ["$total", 30] }}, page] }
}}
],
kids: [{ $skip: (page-1) * 30 }, { $limit: 30 }]
}}